Move C/ObjC interface stuff into separate namespace.
Load frameworks after resetting namespaces.
Rudimentary support for calling C functions directly.

Type information is currently provided in the framework file.
Add cljc.string/split.

The current semantics should match Clojure (i.e. Pattern/split) fairly
closely.

Add an offset argument to pcre-match-offsets so we can avoid unnecessary
copying.
Rename re_pattern, re_match_offsets to pcre_pattern, pcre_match_offsets.
Make split compile on ObjC.

It's still untested.