Removal of libc

[dodomorandi]

Just replaced the libc types with the ones in std.

There is a high chance the crate won't link, because the external calls to malloc, calloc, free and fprintf are still present in the code. However, if speexdfp is used at link time, it should provide a transitive link to libc. libc dependency has been left in Cargo.toml

Should partially solve the first point of #55

[lu-zero]

Would be better to keep the libc crate until the allocations are removed.

[lu-zero]

Thank you!