Small changes to improve compatibility.

[MilchRatchet]

I added const qualifiers to all applicable function parameters. This helps in cases where the input buffer is only available through const qualified pointers.

I also added extern C to all function declarations so that the library can be accessed from C++ code.

As suggested in an issue, I moved some C standard library includes into the source file to avoid cluttering other translation units with unnecessary headers.

Closes #5 Closes #6