Benchmarks += strings, more accurate
Benchmarks += strings, more accurate

Summary:
- No more `int16_t` maps.
- Results now measure cost per lookup.
- Frozen test no longer "cheats" with a smaller key range (accidental)
- String lookup added
- Entry count increased to 1e6

```
============================================================================
thrift/lib/cpp2/frozen/test/FrozenBench.cpp     relative  time/iter  iters/s
============================================================================
----------------------------------------------------------------------------
----------------------------------------------------------------------------
benchmarkLookup(map_f32)                                   374.64ns    2.67M
benchmarkLookup(frozenMap_f32)                   159.93%   234.26ns    4.27M
benchmarkLookup(map_i32)                                   359.70ns    2.78M
benchmarkLookup(frozenMap_i32)                   103.14%   348.75ns    2.87M
benchmarkLookup(map_i64)                                   370.04ns    2.70M
benchmarkLookup(frozenMap_i64)                   111.36%   332.28ns    3.01M
----------------------------------------------------------------------------
benchmarkLookup(hashMap_f32)                                68.05ns   14.70M
benchmarkLookup(frozenHashMap_f32)                80.42%    84.61ns   11.82M
benchmarkLookup(hashMap_i32)                                22.17ns   45.11M
benchmarkLookup(frozenHashMap_i32)                32.25%    68.73ns   14.55M
benchmarkLookup(hashMap_i64)                                26.56ns   37.65M
benchmarkLookup(frozenHashMap_i64)                28.19%    94.21ns   10.61M
benchmarkLookup(hashMap_str)                               129.41ns    7.73M
benchmarkLookup(frozenHashMap_str)                77.64%   166.67ns    6.00M
----------------------------------------------------------------------------
============================================================================
```

Reviewed By: ot

Differential Revision: D7701375

fbshipit-source-id: 6a1b3b9b24bf48da5ec9f7103b1a99d6e56b1230

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