feat(compile): memory statistics in -tstats

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-tstats <file> recorded, for each command of the compiled file, its
location and its time. It now also records, as key=value fields
after these two, its CPU time, the memory it allocated, the size and
the peak size of the major heap after it, and the number of major
collections during it:

1:0 7:23 0.083473 cpu=0.080641 alloc=133.550 heap=33.5 top=33.5 gc=1

Sizes are in MB. They come from Gc.quick_stat, which costs nothing
noticeable. The peak size of the major heap shows which commands make
a file need its memory, and the allocated memory measures the work of
a command independently of the load of the machine.