optimization

[Frank Ch. Eigler]

Hi -

I just committed some work toward PR 2060.  It's a new optimization
sub-pass that runs during the elaboration phase, and aims to eliminate
useless functions, assignments, expressions, variables.  "-u" disables
it; "-v" tells you what it removed.

Plain script code should not be affected (since one only rarely puts
in a deliberately unused side-effect-free expression).  On the other
hand, tapset code that aggressively defines lots of local variables
from $target ones will see a drastic effect on debuginfo fault
tolerance, and quite possibly performance.

Please let me know of any problems you notice.

- FChE