Auto merge of #59693 - nnethercote:64-bit-Spans, r=petrochenkov

Increase `Span` from 4 bytes to 8 bytes.

This increases the size of some important types, such as `ast::Expr` and
`mir::Statement`. However, it drastically reduces how much the interner
is used, and the fields are more natural sizes that don't require bit
operations to extract.

As a result, instruction counts drop across a range of workloads, by as
much as 10% for `script-servo` incremental builds.

Peak memory usage goes up a little for some cases, but down by more for
some other cases -- as much as 18% for non-incremental builds of
`packed-simd`.

The commit also:
- removes the `repr(packed)`, because it has negligible effect, but can
  cause undefined behaviour;
- replaces explicit impls of common traits (`Copy`, `PartialEq`, etc.)
  with derived ones.

r? @petrochenkov

0 files changed, 0 insertions, 0 deletions