A new use-before-definedness checker

[nomeata]

based on the observation in #74 (comment) I wrote a new use-before defined checker. Main differences to the old one (from #84):

• It analyses a block as a whole, first gathering all analysis results on the subcomponents, and then analyzing them, instead of doing it as it traverses the declarations.
• This makes it easier to specify and understand (see the comment above).
• Because of this slightly different structure, I decided to move it out of freevars.ml into its own file. (Note: We are not using freevars.ml any more, besides in some assertions. The compiler uses freevars_ir.ml.)
• This allows it to return a more precise analysis result in the case of a nested block, see test case run/nested-eager.as. Andreas noticed this deficiency of the old one, so hopefully this is nicer.
• The few test cases that I wrote in Use before define check #84 still go through. Yay!
• Because one AST traversal does analysis and checking, we don’t pay the quadratic cost.