Mark string usage as modify safe
Don't randomly resize a buffer outside the managed scope

A GC might happen during a GVL unlock phase, so we can have issues here
with concurrent modification of these buffer.

Also there is no guarantee about the storage mechanism behind a string so we
can't just assume a data structure and muck with that.
Remove now unused method

This code also accessed MRI specific internal, so it wouldn't compile if
this method was still in the source, even though it isn't used now
anymore.
Disable encoding handling for now

This uses rb_econv_* methods that are directly tied to the internal
encoding workings of MRI.
Merge pull request #1 from dbussink/zlib-fixup

Zlib fixup