Remove R: Read bounds on structs

[dtolnay]

None of the R: Read bounds are required. This includes the following types:

• deflate::EncoderReader
• deflate::EncoderReaderBuf
• deflate::DecoderReader
• deflate::DecoderReaderBuf
• gz::EncoderReader
• gz::EncoderReaderBuf
• gz::DecoderReader
• gz::MultiDecoderReader
• gz::DecoderReaderBuf
• gz::MultiDecoderReaderBuf

All of these need R: Read bounds on various impls but not on the structs themselves. Having bounds on the structs is annoying because the bounds transitively infect any structs containing any of these types.

The W: Write bounds in contrast are all required because those types have Drop impls that require the bound. Rust currently requires any bound on a Drop implementation to also be present on the struct.