Allocator for swc.
# Features
- `scoped`: Enable `scoped` mode.
# Modes
## Default mode
In default mode, [crate::boxed::Box] and [crate::vec::Vec] are identical to
the original types in [std].
## Scoped mode
- You need to enable `scoped` feature to use this mode.
In `scoped` mode you can use [FastAlloc] to make [crate::boxed::Box] and
[crate::vec::Vec] very fast.
In this mode, you need to be careful while using [crate::boxed::Box] and
[crate::vec::Vec]. You should ensure that [Allocator] outlives all
[crate::boxed::Box] and [crate::vec::Vec] created in the scope.
Recommened way to use this mode is to wrap the whole operations in
a call to [Allocator::scope].