This patch converts [string compare] so that it
generates INST_STR_EQ;INST_LNOT instead of INST_STR_CMP
when the comparison is going to be used as a boolean
test in the [if], [for] and [while] commands. This
should be faster, particularly on longer strings,
because we don't need to know how the strings differ.
Part of the patch could be construed to form the basis
for a more general optimization framework, but that
probably needs more work to realize...