Properlly deliniate our public API

[GarkGarcia]

We should decide what's going to be part of our public API before releasing. Things that aren't in the public API should be marked with a _ at the start of their names.

As a start, I propose to keep everything other than replace_*_with_*, named_characters, esc_aliases, generate.rl_inputrc and whatever is used by Mathics core outside of the public API. @rocky @mmatera Thoughts?

[GarkGarcia]

Ok, I've realised marking everything with a _ will probably take some time, so here's an alternative solution: I've been writing lot's of documentation both in #11 and in #8 and I plan to merge all of it in a Sphix document. We could then include documentation for functions/classes we consider to be part of our public API in there and leave the rest of the library out of there. That way users aren't told about the existence of things they shouldn't use.