I was thinking that a module called `adler` with a struct called `State32` was a bit redundant (no other structs would exist). We probably want to go the route of reexports though, so perhaps something along the lines of:

* Name the module `adler32`
* Name the struct `Adler32`
* Have the module be private, but the struct reexported at the top of the crate.

I kinda like `compress::Adler32` rather than `compress::checksums::Adler32`, although I suppose with many other checksum algorithms it may be overkill.