Remove cache for TypeExpressionDefinition

[dsherret]

This made more sense when the library was first made, but now it doesn't because there's code generation. Every TypeExpressionDefinition should be unique.

For example, right now every string is a single TypeExpressionDefinition so when you change one string's text it changes all of them.

[dsherret]

Ahhh... well actually now I see this cache is very important because if it doesn't exist it will actually go on forever. I'm thinking for code generation that people should be encouraged to use the manipulation methods ex. setTypeExpression. Problem is that hasn't been implemented yet (#135), but what was found out in this issue should be taken into consideration for sure when implementing that.

Edit: No, not true.

[dsherret]

No, this needs to be fixed. It's really annoying to use this and change one part, but other places update.