parser: Tune stringifyMapKeys

parser: Tune stringifyMapKeys

```bash
benchmark                                               old ns/op     new ns/op     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     3269          3053          -6.61%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        4.79          5.23          +9.19%
BenchmarkStringifyMapKeysIntegers-4                     2707          2320          -14.30%

benchmark                                               old allocs     new allocs     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     16             6              -62.50%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0              0              +0.00%
BenchmarkStringifyMapKeysIntegers-4                     16             6              -62.50%

benchmark                                               old bytes     new bytes     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     1080          1008          -6.67%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0             0             +0.00%
BenchmarkStringifyMapKeysIntegers-4                     1080          1008          -6.67%
```