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/defer_test.v

@@ -0,0 +1,51 @@
+fn foo() string {
+	println('foo()')
+	return 'foo'
+}
+
+fn foo2() string {
+	println('start')
+	defer { println('defer') }
+	defer { println('defer2') }
+	println('end')
+	return foo()
+}
+
+fn test_defer() {
+	assert foo2() == 'foo'
+}
+
+fn set_num(i int, n mut Num) {
+	defer { n.val+=1 }
+	println("Hi")
+	if i < 5 {
+		return
+	} else {
+		n.val+=1
+	}
+}
+
+fn set_num_opt(n mut Num) ?int {
+	defer { n.val = 1 }
+	return 99
+}
+
+struct Num {
+mut:
+	val int
+}
+
+fn test_defer_early_exit() {
+	mut sum := Num{0}
+	for i in 0..10 {
+		set_num(i, mut sum)
+	}
+	println("sum: $sum.val")
+	assert sum.val == 15
+}
+
+fn test_defer_option() {
+	mut ok := Num{0}
+	set_num_opt(mut ok)
+	assert ok.val == 1
+}