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/repl/function.repl

@@ -1,11 +0,0 @@
-fn test() {
-  println('foo')
-}
-test()
-fn test2(a int) {
-  println(a)
-}
-test2(42)
-===output===
-foo
-42