Advanced statement parsing

Currently, Very is very limited in its parsing capabilities when it comes to variable assignments and boolean expressions. Right now, these are limited to the forms VAR a = b <operator> c and IF a <operator> b. If Very is to be productive in any way, this needs to be expanded upon, so that arbitrarily complex statements can be parsed. (Example: VAR %quarter-circumference = (2 * %PI * %radius) / 4)