packed_integers provides a growable array for integer types in the range u1 to u31.
Assume you have a sequence of unsigned integers in the range [0, 100000] that you would like to
hold in memory. That range of values can be represented using 17 bits per integer, since
217 - 1 = 131071. As Rust has no u17 type, you would typically store these values
in a u32 array, wasting 15 bits per integer.
packed-integers helps alleviate this issue by packing these integers at the bit level,
essentially trading time for space.
Where possible, packed-integers mimics the API for Rust's Vec in order to provide a set of
methods you're probably already familiar with.