Basic heap profile support without -prof

Now that constructor info tables contain the name of the constructor,
we can generate useful heap profiles without requiring the whole
program and libraries to be compiled with -prof.  So now, "+RTS -hT"
generates a heap profile for any program, dividing the profile by
constructor.  It wouldn't be hard to add support for grouping
constructors by module, or to restrict the profile to certain
constructors/modules/packages.

This means that for the first time we can get heap profiles for GHCi,
which was previously impossible because the byte-code
interpreter and linker don't work with -prof.