FA/DFA. (The hybrid NFA/DFA uses a special purpose "lazy" state
identifier.)
* [`SmallIndex`] - The internal representation of both a `PatternID` and a
`StateID`. Its purpose is to serve as a type that can index memory without
being as big as a `usize` on 64-bit targets. The main idea behind this type
is that there are many things in regex engines that will, in practice, never
overflow a 32-bit integer. (For example, like the number of patterns in a regex
or the number of states in an NFA.) Thus, a `SmallIndex` can be used to index
memory without peppering `as` casts everywhere. Moreover, it forces callers
to handle errors in the case where, somehow, the value would otherwise overflow
either a 32-bit integer or a `usize` (e.g., on 16-bit targets).
* [`NonMaxUsize`] - Represents a `usize` that cannot be `usize::MAX`. As a
result, `Option<NonMaxUsize>` has the same size in memory as a `usize`. This
useful, for example, when representing the offsets of submatches since it
reduces memory usage by a factor of 2. It is a legal optimization since Rust
guarantees that slices never have a length that exceeds `isize::MAX`.