Added generic string <-> number conversion functions to core::num.

They unify the different implementations that exists in int-template.rs, uint-template.rs and float.rs into one pair of functions, which are also in principle usable for anything that implements the necessary numeric traits. Their usage is somewhat complex due to the large amount of arguments each one takes, but as they're not meant to be used directly that shouldn't be a problem.

Author: Kimundi