This is a very strange design that determines whether initialization
needs to be performed by reassigning a function pointer. A misguided
optimization? Just check explicitly.
The optimizer adds a fair amount of complexity in muparser with no
benefit to fish, since fish is not going to use complicated expressions
or cache parsed expressions.
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.
