This implements option 1 in ninja-build#2462 (comment).
This uses https://github.com/greg7mdp/parallel-hashmap instead of unordered_map for faster ninja execution.
Some more context is in https://crbug.com/346910589.
This makes noop build for chromium
* 12% faster on Linux
* 2% slower on Windows
* 96% faster on macOS
```
$ hyperfine --warmup 1 \
'~/ninja_master -C out/non_component build:buildflag_header_h' \
'~/ninja_faster_hash -C out/non_component build:buildflag_header_h'
Benchmark 1: ~/ninja_master -C out/non_component build:buildflag_header_h
Time (mean ± σ): 5.251 s ± 0.027 s [User: 4.641 s, System: 0.608 s]
Range (min … max): 5.222 s … 5.309 s 10 runs
Benchmark 2: ~/ninja_faster_hash -C out/non_component build:buildflag_header_h
Time (mean ± σ): 4.689 s ± 0.022 s [User: 4.055 s, System: 0.633 s]
Range (min … max): 4.648 s … 4.737 s 10 runs
Summary
~/ninja_faster_hash -C out/non_component build:buildflag_header_h ran
1.12 ± 0.01 times faster than ~/ninja_master -C out/non_component build:buildflag_header_h
```
```
Benchmark 1: C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_master -C out/non_component build:buildflag_header_h
Time (mean ± σ): 13.161 s ± 0.105 s [User: 10.657 s, System: 2.194 s]
Range (min … max): 13.005 s … 13.328 s 10 runs
Benchmark 2: C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_faster_hash -C out/non_component build:buildflag_header_h
Time (mean ± σ): 13.206 s ± 0.210 s [User: 10.653 s, System: 2.194 s]
Range (min … max): 12.951 s … 13.578 s 10 runs
Summary
C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_master -C out/non_component build:buildflag_header_h ran
1.00 ± 0.02 times faster than C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_faster_hash -C out/non_component build:buildflag_header_h
```
```
$ hyperfine --warmup 1 '~/ninja_master -C out/non_component build:buildflag_header_h' '~/ninja_faster_hash -C out/non_component build:buildflag_header_h'
Benchmark 1: ~/ninja_master -C out/non_component build:buildflag_header_h
Time (mean ± σ): 14.360 s ± 0.706 s [User: 9.006 s, System: 1.681 s]
Range (min … max): 12.832 s … 15.297 s 10 runs
Benchmark 2: ~/ninja_faster_hash -C out/non_component build:buildflag_header_h
Time (mean ± σ): 7.340 s ± 0.260 s [User: 6.263 s, System: 0.828 s]
Range (min … max): 6.948 s … 7.799 s 10 runs
Summary
~/ninja_faster_hash -C out/non_component build:buildflag_header_h ran
1.96 ± 0.12 times faster than ~/ninja_master -C out/non_component build:buildflag_header_h
```