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.

vlib/compiler/tests/enum_test.v

@@ -0,0 +1,24 @@
+enum Color {
+	red
+	blue
+	green
+}
+
+fn test_enum() {
+	assert Color.red == Color.red
+	assert Color.blue == Color.blue
+	assert Color.green == Color.green
+
+	assert Color.red == .red
+	assert Color.blue == .blue
+	assert Color.green == .green
+
+	assert Color.red != Color.blue
+	assert Color.red != Color.green
+	assert Color.blue != Color.green
+
+	mut color := Color.red
+	assert color == Color.red
+	color = .green
+	assert color == Color.green
+}