Variables non-recursive and lazy resolution refactor

This replaces !1951 (closed) and includes the change @valentindavid made regarding the addition of Variables.check()

This is pretty much a complete rewrite of _variables.pyx, so that the code should be more comprehensive, and now includes a slightly slower iterative approach to resolving variables in addition to the slightly quicker recursive approach.

The fast approach is tried first and it falls back on the iterative approach, where all error reporting is handled.