compiler: make compiler an ordinary vlib/compiler module

* Move compiler/ under vlib/compiler/ .

* Add a minimal compiler/main.v driver program.

* Cleanup compiler/main.v .

* Make most compiler tests pass again.

* Apply the fix by @joe-conigliaro , so that the rest of the compiler tests are fixed too.

* Thanks to @avitkauskas, now the vlib/vcompiler/tests/str_gen_test.v test does not need to be special cased anymore.

* Reapply @joe-conigliaro fix for vgen.

compiler/tests/interface_test.v

@@ -1,32 +0,0 @@
-struct Dog {
-	breed string
-}
-
-fn (d Dog) speak() {
-	println('dog.speak()')
-}
-
-fn (d Dog) name() string {
-	return 'old gray'
-}
-
-interface Speaker {
-	name() string
-	speak()
-}
-
-interface Speak2er {
-	speak()
-	name() string
-}
-
-fn perform_speak(s Speaker) bool {
-	s.speak()
-	return true
-}
-
-fn test_perform_speak() {
-	d := Dog{}
-	assert perform_speak(d)
-}
-