This commit introduces `--debug-incremental` so that we can start
playing around with incremental compilation while it is still being
developed, and before it is enabled by default.

Currently it saves InternPool data, and has TODO comments for the
remaining things. Deserialization is not implemented yet, which will
require some post-processing such as to build a string map out of
null-terminated string table bytes.

The saved compiler state is stored in a file called <root-name>.zcs
alongside <root-name>.o, <root-name>.pdb, <root-name>.exe, etc. In case
of using the zig build system, these files are all in a zig-cache
directory.

For the self-hosted compiler, here is one data point on the performance
penalty of saving this data:

```
Benchmark 1 (3 runs): zig build-exe ...
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          51.1s  ±  354ms    50.7s  … 51.4s           0 ( 0%)        0%
  peak_rss           3.91GB ±  354KB    3.91GB … 3.91GB          0 ( 0%)        0%
  cpu_cycles          212G  ± 3.17G      210G  …  216G           0 ( 0%)        0%
  instructions        274G  ± 57.5M      274G  …  275G           0 ( 0%)        0%
  cache_references   13.1G  ± 97.6M     13.0G  … 13.2G           0 ( 0%)        0%
  cache_misses       1.12G  ± 24.6M     1.10G  … 1.15G           0 ( 0%)        0%
  branch_misses      1.53G  ± 1.46M     1.53G  … 1.53G           0 ( 0%)        0%
Benchmark 2 (3 runs): zig build-exe ... --debug-incremental
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          51.8s  ±  271ms    51.5s  … 52.1s           0 ( 0%)          +  1.3% ±  1.4%
  peak_rss           3.91GB ±  317KB    3.91GB … 3.91GB          0 ( 0%)          -  0.0% ±  0.0%
  cpu_cycles          213G  ±  398M      212G  …  213G           0 ( 0%)          +  0.3% ±  2.4%
  instructions        275G  ± 79.1M      275G  …  275G           0 ( 0%)          +  0.1% ±  0.1%
  cache_references   13.1G  ± 26.9M     13.0G  … 13.1G           0 ( 0%)          -  0.1% ±  1.2%
  cache_misses       1.12G  ± 5.66M     1.11G  … 1.12G           0 ( 0%)          -  0.6% ±  3.6%
  branch_misses      1.53G  ± 1.75M     1.53G  … 1.54G           0 ( 0%)          +  0.2% ±  0.2%
```

At the end of each compilation with `--debug-incremental`, we end up
with a 43 MiB `zig.zcs` file that contains all of the InternPool data
serialized.

Of course, it will necessarily be more expensive to save the state than
to not save the state. However, this data point shows just how cheap the
save state operation is, with all of the groundwork laid for using a
serialization-friendly in-memory data layout.