topdown: Avoid expensive fmt.Sprintf in getRules.

Most notable improvements:

benchmark                                 old allocs     new allocs     delta
BenchmarkScheduler10x30-8                 91720          91658          -0.07%
BenchmarkConcurrency1-8                   2078243        2077243        -0.05%
BenchmarkConcurrency2-8                   2078463        2077462        -0.05%
BenchmarkConcurrency4-8                   2078813        2077810        -0.05%
BenchmarkConcurrency8-8                   2079289        2078266        -0.05%
BenchmarkConcurrency4Readers1Writer-8     2079039        2078017        -0.05%
BenchmarkConcurrency8Writers-8            2078255        2077258        -0.05%
BenchmarkVirtualDocs10x10-8               415            414            -0.24%
BenchmarkVirtualDocs100x10-8              415            414            -0.24%
BenchmarkVirtualDocs1000x10-8             415            414            -0.24%

Signed-off-by: Teemu Koponen <koponen@styra.com>