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 12% for incremental "check" builds of `script-servo`.
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.
fd7f605365