Optimizer - condense constants into a single string

[tom-leys]

When the compiler (or previous optimization steps in this case) outputs this MLOG

print "Reactor "
print 1
print " in emergency shutdown mode\n"

The optimizer should condense it to

print "Reactor 1 in emergency shutdown mode\n"

This was the result of unrolling a loop which calls a function (now with a constant) and printfs in that function.

The full example is here #136