Refactored evaluate assignment

Moved the code that evaluates an assignment into its own function.
Supplemental control statements for scripting

This implements "for", "break" and "continue" using the existing control
structures inside cs_parser.cpp. They are, after all, just ways of
paraphrasing other control statements. The for statement takes the usual
parameters, but they can be omitted. One caveat is that the for
statement cannot include variable declarations (e.g. int x = 0) for the
initialisation part, only assignments.
Fixed parse error for long form assignments

This changes the way brackets are processed inside evaluate_expression
to fix long form assignments at the end of for loops (e.g. a = a + 2).
Also probably a better way of doing things all 'round.
Fixed import fixups not being moved with a chunk

AGS script has a concept of fixups. I believe it's to facilitate
importing global variables. Memory addresses of these fixups weren't
being altered when a chunk of code was yanked and subsequently written
elsewhere.
Fixed incomplete for loop error messages

These error messages are more descriptive for situations where a user
enters something like:
for()
Merge pull request #121 from gurok/for_break_continue

Supplemental control statements for scripting: for, break, continue