Allow epsilon grammar productions

[jbearer]

We should be able to define grammar productions with no symbols. This is extremely useful. For example, suppose we want to define a possibly empty sequence of symbols:

symbols -> # epsilon
symbols -> <symbol> <symbols>

Design-wise, this is easy. The representation of parse trees as iterable bl:mp objects makes it easy to create a parse tree with no symbols in the production of a macro definition. Implementation-wise, significant changes and additions to the LALR(1) parser will be needed.