async-compression: v0.4.0

• Level::Precise variant now takes a i32 instead of u32.
• Add top level zstd module containing stable zstd crate wrapper types.
• Add ZstdEncoder::with_quality_and_params() constructors.
• Update zstd dependency to 0.12.
• Remove deprecated stream, futures-bufread and futures-write crate features.
• Remove Tokio 0.2.x and 0.3.x support (tokio-02 and tokio-03 crate features).