Turn strings into first class values

[jvdb]

So that the following becomes possible:

SomeStruct {
  somefield: "abc"|"def";
  otherfield: !"abc";
}

[JJWTimmer]

modifiers (terminatedBx) should also work with strings (and multiple values, or the documentation on modifier + multi val should be created).
Also, when allowing strings in not and or, a check if strings are allowed at a certain expression should come in handy.

[jvdb]

I think strings should just return to being real value types at the lowest level, i.e. let the generated code handle strings instead of just bytes. This does present some challenges though (different size strings in expressions means the size is unknown at compile time. Furthermore, even strings that have the same amount of characters may actually be different size due to encoding, so encoding should be done at compile time).