[MLton] getText{Start,End} considered harmful

[Wesley W. Terpstra]

MLton currently uses two functions, GC_getText{Start,End}, to create a giant
lookup table for profiling. These two functions are among the hardest things
to supply when porting MLton. It becomes even harder once shared libraries
are considered, because then one needs to consider which text segment is
intended. After reading the i386 sys v abi specification, I still have no
idea how to do this correctly for shared libraries on elf.

It seems to me that not only are these a portability problem, they are
completely unnecessary. Making a giant lookup table for every address in the
text segment hardly seems like a worthy goal. I imagine the justification is
that it has minimal impact on the performance of the program during handling
of SIGPROF. However, I would contend that a giant lookup table could very
well be more intrusive than just using a binary search of a compressed
(repeated sourceSeqIndexes eliminated) sourceMaps.sourceLabels. Even if the
binary search does have a greater impact, at 1kHZ it will hardly matter.

Of course this leaves open the interpretation of addresses before the first
label and after the last label. As profiling labels are only used for
i386/amd64 native codegens, any address before the first label is simply
unknown. If the native codegens also emitted a label with a no-op at the end
of each assembler file, this would neatly deal with addresses past the last
label: the last label is unreachable anyway, so anything after it is
unknown.

So to sum up: could I just kill these off and use a binary search?
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