Compiler: replace DOM traversal computation

Originally the old trace JIT used a few recursive graph walking
algorithms - which was perfectly reasonable given that the graph
size was capped at a few dozen nodes at most.  These were replaced
with iterative walk order computations  - or at least I thought
they all were.  Missed one of them, which caused a stack overflow
on a pathologically large method compilation.

Renaming of some arena_allocator items for consistency and clarity.
More detailed memory usage logging.  Reworked the allocator to waste
less space when an allocation doesn't fit and a new block must be
allocated.

Change-Id: I4d84dded3c47819eefa0de90ebb821dd12eb8be8