ast, topdown: Improve indexer to understand function args

This commit improves the rule indexer to understand function
args. Function arguments are stored via their argument index, to allow
for different variable names (as re-written by the compiler). So, for

    f(x, y, z) = { x = 10; y = 12; z > 1 }

the indexer will store the rule at

    - args[0]: scalar(10) args[1]: scalar(12)

The callsite-local function arguments are handed off to evalResolver,
which enhances eval's Resolve() method.

Benchmark results:

    name                           old time/op    new time/op    delta
    FunctionArgumentIndex/10-16      23.2µs ± 6%     6.1µs ± 1%  -73.75%  (p=0.008 n=5+5)
    FunctionArgumentIndex/100-16      187µs ± 2%       6µs ± 1%  -96.70%  (p=0.008 n=5+5)
    FunctionArgumentIndex/1000-16    2.23ms ± 3%    0.01ms ± 2%  -99.68%  (p=0.008 n=5+5)

    name                           old alloc/op   new alloc/op   delta
    FunctionArgumentIndex/10-16      20.3kB ± 0%     5.3kB ± 0%  -74.13%  (p=0.008 n=5+5)
    FunctionArgumentIndex/100-16      174kB ± 0%       5kB ± 0%  -96.98%  (p=0.008 n=5+5)
    FunctionArgumentIndex/1000-16    1.76MB ± 0%    0.01MB ± 0%  -99.70%  (p=0.008 n=5+5)

    name                           old allocs/op  new allocs/op  delta
    FunctionArgumentIndex/10-16         296 ± 0%        91 ± 0%  -69.26%  (p=0.008 n=5+5)
    FunctionArgumentIndex/100-16      2.29k ± 0%     0.09k ± 0%  -96.02%  (p=0.008 n=5+5)
    FunctionArgumentIndex/1000-16     22.1k ± 0%      0.1k ± 0%  -99.59%  (p=0.008 n=5+5

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

Co-authored-by: Stephan Renatus <stephan.renatus@gmail.com>
Co-authored-by: Torin Sandall <torinsandall@gmail.com>