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/mut_test.v

@@ -0,0 +1,52 @@
+struct A {
+pub mut:
+	v []int
+}
+
+struct B {
+pub mut:
+	a []A
+}
+
+fn foo(b int, a mut []int) {
+	a[0] = 7
+	//a << 4
+}
+
+fn test_mut() {
+	mut numbers := [1,2,3]
+	foo(7, mut numbers)
+	assert numbers.len == 3
+	// TODO bring back once << works with mutable args
+	//assert numbers.len == 4
+	//assert numbers[0] == 7
+	//assert numbers[3] == 4
+	println(numbers)
+	n := 1
+	mut b := &n
+	*b = 10
+	//mut b := mut a
+	//b = 10
+}
+
+fn test_mut_2() {
+	zero := 0
+
+	mut b := B{}
+	b.a << A{}
+
+	b.a[0].v = [9, 8, 7]
+
+	b.a[0].v << 6
+	b.a[zero].v << 5
+
+	b.a[0].v[zero] = 3
+	b.a[0].v[b.a[zero].v[zero]] += 1b.a[0].v[b.a[0].v[zero]] += 1
+
+	assert b.a[0].v.len == 5
+	assert b.a[0].v[0] == 3
+	assert b.a[0].v[1] == 8
+	assert b.a[0].v[2] == 7
+	assert b.a[0].v[3] == 8
+	assert b.a[0].v[4] == 5
+}