Replace internal buffer in decoder with `BufRead`

[Kixunil]

The DecoderReader used an internal buffer for reading which came with a number of disadvantages, such as:

• Needless copying from in-memory readers (slices)
• Double-buffering already-buffered readers
• The bytes are lost when dropping or calling into_inner
• BUF_SIZE is not configurable
• std::io::BufReader has access to some unstable optimizations which this crate cannot use
• Reinvents the wheel; there already is std::io::BufReader

This change removes it and requires BufRead instead. Decoding is implemented on top of fill_buf for larger chunks with fallback to small, stack-allocated buffer for tiny chunks that may appear at boundaries.

To resolve borrowing problems decode_to_buf had to be removed which also enabled decoding bytes directly into internal buffer when the buffer to be filled is small rather than into a temporary buffer which is then copied.

This improves performance of reading by around 7-22% on my machine.

Closes #230

[marshallpierce]

Thanks for the contribution. It will probably take me some time to review it, but I'll get to it when I can.