A type for storing fixed-size boolean arrays densely to optimize space

Inspired by C++'s std::vector<bool> implementation

# Overview

`compact_bitset` adds a new container type, `CompactBitset`. A 
`CompactBitset`
stores boolean variables so that one bit is used per `bool`. It is 
designed for cases
where a boolean array is necessary, but `Vec<bool>` or a slice is 
undesired due to
space requirements. The `CompactBitset` reduces size used per `bool` to an 
eighth of
what it would otherwise be.

# Details

`CompactBitset` uses the proxy types `BitRef` and `BitRefMut` for indexing 
operations.

These two types deref to `bool` and implement comparison to `bool` as well 
as each other.

Writing to a `BitRefMut` alters the bit that it symbolically references in 
the `CompactBitset`.

`BitRef` and `BitRefMut` contain references to their parent 
`CompactBitset`. Thus, any number of
`BitRef`s can be borrowed from a `CompactBitset`, but only 1 `BitRefMut` 
may be active at a time.
This allows `BitRef` and `BitRefMut` to take advantage of the safety 
guarantees of references.

`BitIter` allows the user to immutably iterate the `CompactBitset` via a 
wrapper around a `BitRef`
with its index incremented. It contains all standard `Iterator` 
functionality.

`BitIterMut` may be on the roadmap, but a current implementation has not 
successfully been created
without use of `unsafe`.