This change provides a common convention for using allocator-dependent
features with:
#![cfg(feature = "alloc")]
When available, `Vec` is imported consistently as `prelude::Vec`, which
means modules that need access to `Vec` can simply do:
use prelude::*;
and if an allocator is available, `Vec` will be in the crate prelude.
This allows all `alloc` vs `std` gating to be handled in `lib.rs`,
`build.rs`, and `prelude.rs` so the rest of the codebase doesn't have to
do any gating whatsoever.
Each backend can now be selected by an individual feature:
- `u32_backend` for `backend::u32`;
- `u64_backend` for `backend::u64`;
- `avx2_backend` for `backend::avx2`;
The `u64_backend` is selected by default, since most people use X64 and we have
no way to select based on target (see discussion in #126). However, these
changes mean that it is possible to select the backend explicitly, and if we
had the ability to select target-default features, we could do so easily.
