feat: Impl BufReader for std::io::Read

[cpubot]

This PR adds a new Reader impl, BufReader which adds Reader support for any std::io::Read.

The implementation is generally what one would expect from a buffered reader implementation. The key nuance to pay attention to is the as_trusted_for implementation. Specifically, BufReader's Trusted implementation uses the following enum:

pub enum TrustedBufReader<'a, 'b, R: ?Sized> {
    WithinCapacity(TrustedSliceReader<'a, 'b>),
    ExceedsCapacity(BufReaderMut<'b, R>),
}

This type captures two possible scenarios when requesting a trusted window:

1. The requested window is within the buffer capacity.
2. The requested window is larger than the buffer capacity.

In the first case, we can provide a TrustedSliceReader, which achieves the desired trusted semantics where bounds checks are eliminated, and is very performant.
In the second case, we provide a BufReaderMut, which effectively acts as a typical non-trusted BufReader.

An alternative implementation could reallocate the buffer to accomodate the requested window size, but this route provides simplicity, predictability, and user-tuneability. In particular, rather than implicitly reallocating on behalf of the user, one may simply tune capacity for a specific use case.

See #53 for a longer discussion on these points.

Closes #50