Optimize Vec push by preventing address escapes

This change makes RawVecInner non-generic over the allocator, allowing it
to be Copy. The allocator is moved to RawVec itself. Key optimizations:

- RawVecInner is now Copy (no allocator field)
- grow_one uses ptr::read/ptr::write to copy allocator to a temporary,
  preventing &self from escaping through &dyn Allocator parameter
- Drop::drop similarly copies to temporaries before deallocating
- deallocate takes self by value instead of &mut self
- All these functions are #[inline(always)]

This allows LLVM to keep Vec fields (cap, ptr, len) in registers during
push loops instead of storing/loading from memory every iteration.

Benchmark results (push with pre-allocated capacity):
- 100 elements:   1.74x faster
- 1000 elements:  1.87x faster
- 10000 elements: 2.41x faster

Secondary benefit: grow_one_impl and other growth functions use &dyn Allocator,
so they are compiled once in libstd rather than monomorphized per allocator type.

Preserves const compatibility with the const_heap feature by using generics
for the const allocation path while using &dyn Allocator for runtime paths.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gerben-stavenga