compiler: add support for type parameters (aka generics)

...that was surprisingly easy.

testdata/generics.go

@@ -0,0 +1,14 @@
+package main
+
+func main() {
+	println("add:", Add(3, 5))
+	println("add:", Add(int8(3), 5))
+}
+
+type Integer interface {
+	int | int8 | int16 | int32 | int64
+}
+
+func Add[T Integer](a, b T) T {
+	return a + b
+}

testdata/generics.txt

@@ -0,0 +1,2 @@
+add: 8
+add: 8