Added free-variable and free-call computation of ANF IL.

This will be used in the lambda lifter. The free-call computation
collects the set of calls to user defined procedures that are
'free' (not found in the expression by a LET_REC).
Added a first version of the lambda lifter.

The lambda lifting argument is complicated because
of the presence of mutually recursive functions. The
algorithm is nicely described in

Johnsson, Thomas
'Lambda Lifting: Transforming Programs to Recursive Equations'

In any case, the simplest test case (test/good/nested0.tig)
compiles successfully.