Auto merge of #119791 - Mark-Simulacrum:cut-leb128, r=cjgillot

Remove a large amount of leb128-coded integers

This removes ~41%[^1] of the leb128-encoded integers serialized during libcore compilation by changing enum tags to opportunistically use `u8` where feasible instead of the leb128 coding via `usize`.

This should have effectively zero impact on metadata file sizes, since almost all or all enum tags fit into the 7 bits available in leb128 for single-byte encodings. Perf results indicate this is basically neutral across the board except for an improvement in bootstrap time.

[^1]: More than half the remaining integers still fit into <= 128, so the leb128 coding still makes sense. 32% are zero, and 46% are <= 4.

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