This causes unnecessary calls to `grow` when the allocation would fit
exactly in the remaining space.

They are pointless. No reasonable allocator will be able to satisfy a
`reserve_in_place` request that *doubles* the size of an allocation when
dealing with allocations that are 4 KiB and larger.

Just to be sure, I confirmed on Linux that the `reserve_in_place` calls
never succeed.

(Note however that the `reserve_in_place` call for `DroplessArena::grow`
did occasionally succeed prior to the off-by-one fix in the previous
commit, because we would sometimes do a `reserve_in_place` request for
the chunk's current size, which would trivially succeed!)

Also remove a now-unnecessary `placement` argument.

- Use `len` more consistently for the number of elements in a vector,
  because that's the usual name.
- Use `additional` more consistently for the number of elements we want
  to add, because that's what `Vec::reserve()` uses.
- Use `cap` consistently rather than `capacity`.
- Plus a few other tweaks.

This increases consistency and conciseness.

It already has a Debug implementation.

This flag looks like it's been removed in LLVM 10, so this removes rustc
unconditionally passing the flag.

…ace, r=Amanieu

Remove `RawVec::reserve_in_place`.

And some related clean-ups.

r? @oli-obk

remove trivial calls to mk_const

similar to rust-lang#72754

changed *nix to Unix-like

…an-DPC

Clean up E0637 explanation

r? @Dylan-DPC

Remove allow missing_debug_implementations for MaybeUninit

It already has a Debug implementation.

…=nikomatsakis

rustc: Remove the `--passive-segments` LLD flag on wasm

This flag looks like it's been removed in LLVM 10, so this removes rustc
unconditionally passing the flag.