Figure out what the minimal public API actually is

[Fuco1]

I suspect that sp-get-string, sp-get-sexp, sp-get-thing, sp-get-symbol should be enough. Ideally confirm this and re-implement everything in terms of these primitives.

We will need to move all the others into a private namespace and discourage their use in any other place than these four functions.

This will allow us to overload these for specific major modes when a better implementation is available.