Replace Type::TypeVariable, Type::PolymorphicInteger, and all other type variables with one kind

[jfecher]

Problem

As the typechecker grows it can be useful to have different kinds of type variables. E.g. variables that can bind to anything, or only integer types, or integer and field types, etc. However, for these cases we currently have to create an entirely new variant of Type each time and implement most cases the same way.

Happy Case

We should only have Type::TypeVariable, and it should contain an internal enumeration TypeVariableKind detailing what kind of type variable it is. This will allow us to add and work with type variables more easily without bloating our Type representation. Example of this enum:

enum TypeVariableKind {
    /// Can bind to any type
    Normal,
    /// Can only bind to integer types (not including fields). Used for comparison and bitwise operations
    Integer,
    /// Can bind to integer or field types. Used for most arithmetic operations.
    IntegerOrField,
    /// Can only bind to constant integers e.g. array sizes.
    Constant,
}

Alternatives Considered

No response

Additional Context

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Would you like to submit a PR for this Issue?

No

Support Needs

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[kevaundray]

@f01dab1e feel free to take this on