feat: Support generic types

[klauspost]

This adds support for generic types.

Say you have type Foo[T any] struct, you must add a constraint that it can be handled properly. The constraint is added with a built-in type Foo[T any, _ msgp.RTFor[T].

If T supports serialization this will work. Multiple types can be added, but each must have the constraint.

Nested of types are supported. For example:

type Foo[T any, P msgp.RTFor[T]] struct {
    X  Bar[T, P] `msg:",allownil"`
}

type Bar[T any, _ msgp.RTFor[T]] struct {...}

Notice how P can be used for a nested generic type.

Limitations

• Types cannot be primitives since they must support marshalling. It can however be an type Int64 int with generation should still work
• Since we cannot reliably create instances with arbitrary types there are no tests.
• The msgp.RTFor[T] type shouldn't be used for types in the struct since new instances cannot be created and can cause a runtime crash.
• There will be warnings like: warn: generics.go: GenericTest: A: possible non-local identifier: T.
• Some directives may exhibit unexpected behaviour. Not tested too deeply.

Example

// MyStruct can be instantiated with any type T that supports marshaling.
// T must be the base type and not a pointer to work. 
type MyStruct[T any, P msgp.RTFor[T]] struct {
	// Direct use of T
	A  T
	C  []T
	D  map[string]T

	// T used for another generic.
	E  MyStruct2[T, P, string]
	F  []MyStruct2[T, P, string]
	G  map[string]MyStruct2[T, P, string]

	// Same with pointers
	AP *T
	CP []*T
	DP map[string]*T
	EP *MyStruct2[T, P, string]
	FP []*MyStruct2[T, P, string]
	GP map[string]*MyStruct2[T, P, string]
}

// MyStruct2 can be used inside MyStruct.
type MyStruct2[T any, P msgp.RTFor[T], B any] struct {
	A T
}

[klauspost]

Ping @philhofer - Any objections to this being merged? Do you see a better approach?