add checksum functions and tobinary functions

[starwing]

• port crc32 and adler32 functions to lua.
• add a function convert int32 checksum to string.

[starwing]

Okay, I have added the documents and test cases, and remove the tobinary function :)

[brimworks]

Sorry for being slow on the uptake here... after thinking about this a bit, I'm proposing a function factory interface that more closely mirrors the compression interface:

f94aabf

Specifically you can do this:

        -- All in one call:
        local csum = zlib.crc32()("one two")

        -- Multiple calls:
        local compute = zlib.crc32()
        compute("one")
        assert(csum == compute(" two"))

        -- Multiple compute_checksums joined:
        local compute1, compute2 = zlib.crc32(), zlib.crc32()
        compute1("one")
        compute2(" two")
        assert(csum == compute1(compute2))

Feedback is welcomed.