Unhack the parser and introduce YieldExpression that emits the
equivalent set of opcodes that were emitted by bunch of
expressions/statements generated by parser before.
YieldExpression expects evaluation stack to contain just the value
being yielded, so {,List}AssignmentExpression need to evaluate RHS
first. The previous code had the same behavior.
This will let us consolidate continuation-related opcodes and make
them less tied with continuation objects.
jan