feat: Implement some basic statistics via the --stats flag

[davidlattimore]

No description provided.

[marxin]

Note so far we've been using RUST_LOG=metrics=debug for the metrics:

2026-01-10T21:02:50.946213Z DEBUG metrics: input_sections loaded_bytes=22416 loaded_compressed_bytes=0 decompressed_bytes=0
2026-01-10T21:02:50.947930Z DEBUG metrics: merge_strings section=`.rodata` string_count=177 byte_size=2363 input_string_count=180 input_string_byte_size=2368 output_map_overflow=61
2026-01-10T21:02:50.947975Z DEBUG metrics: merge_strings section=`.comment` string_count=2 byte_size=28 input_string_count=1432 input_string_byte_size=20048 output_map_overflow=0
2026-01-10T21:02:51.076890Z DEBUG metrics: resolved relocations section="`.text`" relocations=939735
2026-01-10T21:02:51.076927Z DEBUG metrics: resolved relocations section="`.init`" relocations=1
2026-01-10T21:02:51.076930Z DEBUG metrics: resolved relocations section="`.data`" relocations=428
2026-01-10T21:02:51.076934Z DEBUG metrics: resolved relocations section="`.tdata`" relocations=1
2026-01-10T21:02:51.076937Z DEBUG metrics: resolved relocations section="`.data.rel.ro`" relocations=55094
2026-01-10T21:02:51.076939Z DEBUG metrics: resolved relocations section="``.fini_array`` (secondary)" relocations=2
2026-01-10T21:02:51.076942Z DEBUG metrics: resolved relocations section="``.init_array`` (secondary)" relocations=429

Our ambition should be to have a unified approach how to present the metrics about a linking invocation.

[davidlattimore]

Yeah, you're right, it would be best to just have one lot of metrics. I'll have a think about how to proceed, but will probably just close this PR. In the longer run, it might be good to have an easy to remember flag like --stats. I also like how GNU ld's --stats flag can take a filename as an argument and also has an environment variable that if set, writes the stats to the specified file. That allows people to write metrics to a file without needing to adjust the linker command-line.