A minimal implementation of Adler32 for Rust.
This provides the simple method adler32(), that exhausts a Read and computes the Adler32 hash, as well as the RollingAdler32 struct, that can build a hash byte-by-byte, allowing to 'forget' past bytes in a rolling fashion.
The adler32() function has been translated (as accurately as I could manage) from the zlib implementation.