Enhancements related to loops

[simonjwinwood]

• [ ] See #317
• [ ] Static analysis to give loop bounds would let us figure out reasonable unfolding counts.
• [ ] Handle concat in MuxValue.hs (or remove Concat from DDL/replace by a prelude fun)
• [ ] Investigate uses of explodeSequence in MuxValue.hs for perf. improvements
• [ ] Investigate fan-out for loops (i.e., { xs = Many P; map (x in xs) ... } the map relies only on the value, we could get multiple bodies)
• [ ] Create a SMT function for loop bodies which are unfolded.
• [ ] Search for satisfying lengths for unrolled loops (requires linking unrolled loops and their use to extend list)
• [ ] Split out nested loops where the inner loop body doesn't effect the outer loop body
• [ ] Only get the minimum solver model needed when pooling loops (annotate pathbuilder loop nodes with the symbolicmodel)