Remove 'register' keyword from st.h in C API

'register' keyword is meaningless in modern compilers and it causes
deprecation warnings in C++11 compilers.
Merge pull request #3111 from seanchas116/remove-register-from-st-h

Remove 'register' keyword from st.h in C API
Removed README notice of kwargs being unsupported

These were implemented a while ago.
Make JIT inline cast_for_multi_block_arg correctly

(Bug originally found in rbx-1.3.3)

JIT was not inlining arguments correctly when given more than one
argument, and would end up yielding cUndef (Object:0x22) instead.

See https://gist.github.com/sshao/7eecc87458d3e55d7f5d
(run on rbx-1.3.3) for examples.