lib/src/analysis_reftypes.rs: use DFS instead of fixpoint iteration

[julian-seward1]

In an upcoming commit (PR #80) to support reftypes, fn do_reftypes_analysis uses a fixpoint iteration to compute the transitive closure of all VirtualRanges and RealRanges to which reftype-ness can propagate. This is inefficient and unnecessary (and, as implemented, just plain incorrect). This should be rewritten to use a simple (set of) depth-first-searches to compute the transitive closure.