To help remove exception handling, we will need to have a type that
has visibility into both ParserError and value_type. We're going to
put this type in muParserDef.h. Remove the error header and fold its
contents into muParserDef.h.
This was a silly data structure that didn't carry its weight.
Replace it with a wrapper around std::vector that doesn't explicitly
throw exceptions. It's unclear if muparser relied on the exception
throwing behavior of ParserStack, and it seems there's no way to find
out except removing it and seeing what breaks.
The tests pass for what that's worth!
