Generics: Instantiation of the list of generic structs is generated w/out generics

[vgarvardt]

Generics support in the new version is great, but I've found an issue with the generated code for a list case: if you add a list type with generics then its instantiation in the generated code is missing generics definition.

The easiest way to reproduce the issue is to add

type GenericTest3List[A, B any, AP msgp.RTFor[A], BP msgp.RTFor[B]] []GenericTest3[A, B, AP, BP]

to the https://github.com/tinylib/msgp/blob/master/_generated/generics.go and generate the code - DecodeMsg and UnmarshalMsg are generated with the following instantiation code:

	if cap((*z)) >= int(zb0002) {
		(*z) = (*z)[:zb0002]
	} else {
		(*z) = make(GenericTest3List, zb0002)
	}

while it should be

	if cap((*z)) >= int(zb0002) {
		(*z) = (*z)[:zb0002]
	} else {
		(*z) = make(GenericTest3List[A, B, AP, BP], zb0002)
	}

[klauspost]

Yeah, support is for structs for now. I suspect maps will be the same thing. Keeping it on the TODO list.

[klauspost]

Take a look at #418

[vgarvardt]

Take a look at #418

Looks great, thank you!