style: rename alloc to allocator

[Denimworld12]

Closes #640.

Renamed the identifier everywhere it appears:

• the RawSmallVec::allocator and DropDealloc::allocator fields
• the allocator parameters on new, new_inline, new_heap, new_in, with_capacity_in and try_with_capacity_in
• every use site

Deliberately left alone, since alloc there is not this identifier:

• paths into the alloc crate — extern crate alloc, use alloc::alloc::{…}, alloc::vec::Vec, alloc::alloc::handle_alloc_error, and the [dealloc]: alloc::alloc::GlobalAlloc::dealloc doc link
• three comments using "alloc" as prose, e.g. // and we never alloc null pointer.

Both fields are internal — RawSmallVec::allocator is pub(crate) and DropDealloc is a private struct — so this isn't a public API change. Parameter names are visible in docs but don't affect callers.

Checks

Run locally on stable:

• cargo build — clean
• cargo test — 71 + 8 pass
• cargo clippy --all-targets -- -D warnings — clean

I didn't run cargo fmt. rustfmt.toml uses nightly-only options, so stable rustfmt reformats files across the whole repo, including ones untouched here. Instead I checked that no changed line exceeds max_width = 100, since error_on_line_overflow = true and the rename adds four characters per occurrence.

--all-features and --no-default-features need nightly (#![feature] on specialization / allocator_api); both fail identically on upstream/v2 without this change, so I left them to CI.

[alejandro-vaz]

looks good but I'd want #637 merged first which is going to change a bunch of this

[alejandro-vaz]

I'm drafting the PR just to mentally mark it as blocked

[alejandro-vaz]

I think I fucked it up when merging

[Denimworld12]

@alejandro-vaz Do you want me to rebase it onto the current v2 and resolve the conflicts?

[alejandro-vaz]

yeah do it, more conflicts have appeared

[alejandro-vaz]

what happened??