Bind/wrap st_* functions

[malept]

I'd like to use some of the st_* functions because I'm porting an existing C extension at work. In particular, I want to use st_lookup so I can implement Hash.has_key. I have a branch that adds initial bindings for it at https://github.com/malept/ruby-sys/tree/st-functions. My progress has subsided partially because I don't think my bindings are quite correct (I think I tried to implement has_key and I got a weird segfault...I'd have to look again).