Ready-made C function wrappers that can be used by Rubex programs

Using the C API functionality created by https://github.com/SciRuby/rubex/issues/19 it should be possible to create Rubex wrappers over C functions from C standard libraries like `stdlib.h` and `math.h`. This functionality can then be easily used in other Rubex programs using the `require_rubex` construct. It will build upon https://github.com/SciRuby/rubex/issues/48.