Export a multi-entry SSA2 functor

A new functor, MeSsa2, now exports a multi-entry SSA2 IL.  This is the
culmination of:

b18ade8 Convert SSA2 Simplify to the multi-entry SSA2 IL (partially)
ef08b4f Convert the SSA2 Shrink functor to the multi-entry SSA2 IL
18a4ab5 Convert the SSA2 PrePasses functor to the multi-entry IL
35d9463 Convert the SSA2 type-checker to the multi-entry SSA2 IL
0abb7bb SSA2: Convenience functions for finding 'main'
0dc5617 Convert SSA2 Analyze to the multi-entry SSA2 IL
f0c14c5 Copy SSA2 passes needed to convert ssa2.fun to multi-entry
ec83d53 SSA2: Make 'main' a FuncEntry, not a Func
226a879 Convenience functions for SSA2 FunctionEntry
ca444d0 Fix dominatorForest calculation for SSA2
229c90e Add multi-entry functions to the SSA2 IL