ast: avoid extra work parsing objects vs obj comprehensions (#3726)

Now we're only caching what we need in a linked-list form. Anything
we don't need anymore is dropped, and we are thus more space-efficient
than previous approaches.

The idea is that when an object or array is pushed to the cache,
its elements, which had been parsed and cached before, will be
dropped. This is to support the case we're interested in improving:

  * object, or set or object/set comprehension
  * array or array comprehension

```
name                                 old time/op    new time/op    delta
ParseModuleRulesBase/1-16              4.32µs ± 4%    4.80µs ±19%  +11.13%  (p=0.008 n=5+5)
ParseModuleRulesBase/10-16             30.5µs ± 4%    35.6µs ± 4%  +16.67%  (p=0.008 n=5+5)
ParseModuleRulesBase/100-16             302µs ± 2%     340µs ± 2%  +12.56%  (p=0.008 n=5+5)
ParseModuleRulesBase/1000-16           3.38ms ± 6%    3.45ms ± 2%     ~     (p=0.548 n=5+5)
ParseStatementBasicCall-16             2.70µs ± 8%    3.07µs ± 6%  +13.74%  (p=0.008 n=5+5)
ParseStatementMixedJSON-16             78.6µs ± 1%   101.0µs ± 4%  +28.43%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1-16         2.20µs ± 5%    2.43µs ± 8%  +10.52%  (p=0.008 n=5+5)
ParseStatementSimpleArray/10-16        7.61µs ± 1%    8.74µs ± 1%  +14.78%  (p=0.016 n=4+5)
ParseStatementSimpleArray/100-16       58.1µs ± 1%    73.0µs ± 3%  +25.65%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1000-16       555µs ± 4%     759µs ± 1%  +36.76%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x1-16     2.93µs ± 3%    3.52µs ± 3%  +20.09%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x1-16     8.10µs ± 2%   10.24µs ± 4%  +26.46%  (p=0.008 n=5+5)
ParseStatementNestedObjects/10x1-16    14.0µs ± 2%    18.2µs ± 3%  +29.72%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x5-16     7.87µs ± 2%   10.67µs ± 5%  +35.57%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x10-16    13.6µs ± 3%    19.4µs ±10%  +41.96%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x5-16     4.99ms ± 5%    6.94ms ± 2%  +39.08%  (p=0.008 n=5+5)
ParseBasicABACModule-16                46.4µs ± 2%    54.7µs ± 2%  +17.80%  (p=0.008 n=5+5)

name                                 old alloc/op   new alloc/op   delta
ParseModuleRulesBase/1-16              3.97kB ± 0%    4.88kB ± 0%  +23.00%  (p=0.008 n=5+5)
ParseModuleRulesBase/10-16             26.0kB ± 0%    32.4kB ± 0%  +24.54%  (p=0.029 n=4+4)
ParseModuleRulesBase/100-16             252kB ± 0%     314kB ± 0%  +24.22%  (p=0.008 n=5+5)
ParseModuleRulesBase/1000-16           2.56MB ± 0%    3.17MB ± 0%  +23.77%  (p=0.008 n=5+5)
ParseStatementBasicCall-16             3.02kB ± 0%    3.63kB ± 0%  +20.13%  (p=0.008 n=5+5)
ParseStatementMixedJSON-16             51.9kB ± 0%    86.3kB ± 0%  +66.15%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1-16         2.86kB ± 0%    3.46kB ± 0%  +21.29%  (p=0.008 n=5+5)
ParseStatementSimpleArray/10-16        5.86kB ± 0%    9.20kB ± 0%  +57.10%  (p=0.008 n=5+5)
ParseStatementSimpleArray/100-16       38.4kB ± 0%    69.1kB ± 0%  +80.06%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1000-16       390kB ± 0%     694kB ± 0%  +78.07%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x1-16     3.66kB ± 0%    4.57kB ± 0%  +24.95%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x1-16     6.44kB ± 0%    9.78kB ± 0%  +51.93%  (p=0.008 n=5+5)
ParseStatementNestedObjects/10x1-16    10.0kB ± 0%    16.4kB ± 0%  +63.77%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x5-16     8.51kB ± 0%   11.86kB ± 0%  +39.29%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x10-16    14.6kB ± 0%    20.9kB ± 0%  +43.85%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x5-16     3.42MB ± 0%    5.79MB ± 0%  +69.53%  (p=0.008 n=5+5)
ParseBasicABACModule-16                35.1kB ± 0%    44.2kB ± 0%  +25.99%  (p=0.016 n=5+4)

name                                 old allocs/op  new allocs/op  delta
ParseModuleRulesBase/1-16                80.0 ± 0%      89.0 ± 0%  +11.25%  (p=0.008 n=5+5)
ParseModuleRulesBase/10-16                600 ± 0%       663 ± 0%  +10.50%  (p=0.008 n=5+5)
ParseModuleRulesBase/100-16             5.83k ± 0%     6.43k ± 0%  +10.34%  (p=0.008 n=5+5)
ParseModuleRulesBase/1000-16            58.0k ± 0%     64.1k ± 0%  +10.34%  (p=0.008 n=5+5)
ParseStatementBasicCall-16               47.0 ± 0%      53.0 ± 0%  +12.77%  (p=0.008 n=5+5)
ParseStatementMixedJSON-16              1.02k ± 0%     1.36k ± 0%  +33.33%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1-16           36.0 ± 0%      42.0 ± 0%  +16.67%  (p=0.008 n=5+5)
ParseStatementSimpleArray/10-16           112 ± 0%       145 ± 0%  +29.46%  (p=0.008 n=5+5)
ParseStatementSimpleArray/100-16          837 ± 0%      1140 ± 0%  +36.20%  (p=0.008 n=5+5)
ParseStatementSimpleArray/1000-16       8.05k ± 0%    11.05k ± 0%  +37.31%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x1-16       49.0 ± 0%      58.0 ± 0%  +18.37%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x1-16        121 ± 0%       154 ± 0%  +27.27%  (p=0.008 n=5+5)
ParseStatementNestedObjects/10x1-16       207 ± 0%       270 ± 0%  +30.43%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x5-16        125 ± 0%       158 ± 0%  +26.40%  (p=0.008 n=5+5)
ParseStatementNestedObjects/1x10-16       220 ± 0%       283 ± 0%  +28.64%  (p=0.008 n=5+5)
ParseStatementNestedObjects/5x5-16      71.1k ± 0%     94.5k ± 0%  +32.95%  (p=0.008 n=5+5)
ParseBasicABACModule-16                   841 ± 0%       931 ± 0%  +10.70%  (p=0.008 n=5+5)
```

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>