Heap Profiling
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This commit adds simple hp2ps-style heap profiling.  Just the '-h'
option is supported so far.

Heap profiles will show the top two cost centres on each cost centre
stack.

Time/Alloc profiles have been extended to aggregate the costs on a
per-cost-centre basis and show the table of highest consuming cost
centres before the full table of cost centre stacks.

Profiles work best if '-caf-all' is used, especially when using
one of the '-auto' flags.