Support of union and type approximation in generics

[zcolleen]

I discovered that in last version of minimock (v3.3.7) when we upped version of gowrap we broke generation of interfaces with generic unions. It happened because of this pr #84 . So here i introduce support of generic types with union, e.g.

type genericInlineUnionWithManyTypes[T int | float64 | string] interface {
	Name(T)
}

and with type approximations , е.g.

type generic[T ~int] interface {
	Name(T)
}

and their combinations.

I also removed panic of unsupported generic type, because if we encounter unsupported generic type we should be able at least to finish generation without panicing.