Rework igor.struct to support dynamic structures.

Rework igor.struct to support dynamic structures.

Between binarywave and record.variables, there was a good deal of
parsing that was conditional on previously parsed data.  Rather than
continue writing spaghetti code to handle each specific case, I've
taken a stab at a general framework for updating structures during
parsing (DynamicStructure and DynamicField).  The docstrings should
explain how they work.  The implementation still has a few holes, but
it works on each of the files in my test suite.