Expose standalone expression parsing via Frontc

[sim642]

This is currently on top of #96, but could be rebased if necessary.

Since Formatcil is not intended for parsing standalone expressions from user input and is very fragile (#94, #95), then this is an attempt to provide a proper alternative. It exposes Frontc and Cabs2cil functions for parsing just an expression from a given string. This allows expressions to be parsed using the normal parsing pipeline with all the usual behavior (correct typing, implicit casts, etc).

This is a more reliable alternative to be used in goblint/analyzer#745.

This might still be buggy, because doPureExp, which is at the center of this, uses at least 30 global variables (I might have missed some). Here I just require env and genv to be given such that the expression can be converted to CIL in the correct function's context (with all the variables referring to a particular function's locals). As to rest of the globals, there's no way to reconstruct their content at a particular function as they were when the entire file was parsed, so this might still break in surprising ways, but it should already be better than Formatcil.

[sim642]

Weird, merging #96, which this was on top of, didn't for some reason cause the base of this PR to change to develop automatically, so it still merged to the check-exp branch. I'll merge it to develop directly myself then.

[sim642]

Oh, I know: this repository didn't have "Automatically delete head branches" enabled like the analyzer repository. I enabled it here as well and deleted all the branches corresponding to merged PRs to clean things up.