Implement global integer interval analysis

[alastairreid]

In theory, the ASL constraint feature allows programmers to annotate functions with their range.
In practice, the constraints needed on some functions are quite noisy and not likely to help readers of the specification.

Proposal:

1. Build a simple interval analysis that can be applied to functions to infer their return type intervals.
2. Visit functions in reverse dependency order (i.e., leaves first) using the inferred intervals for callees functions already visited to infer the return type interval for each layer of the tree.

Note that this will not be sufficient to infer intervals on function arguments. To do that, we need a top-down analysis that propagates interval information from callers to callees.
The best solution would be a conventional global analysis that refines intervals on all functions at once.
But it may be sufficient to write this as a separate analysis and alternate the bottom up and top down analyses a few times?