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/string_interpolation_test.v

@@ -0,0 +1,29 @@
+
+fn test_excape_dollar_in_string() {
+  i := 42
+
+  assert '($i)' == '(42)'
+  assert '(\$i)'.contains('i') && !'(\$i)'.contains('42')
+  assert !'(\\$i)'.contains('i') && '(\\$i)'.contains('42') && '(\\$i)'.contains('\\')
+  assert '(\\\$i)'.contains('i') && !'(\\\$i)'.contains('42') && '(\\$i)'.contains('\\')
+  assert !'(\\\\$i)'.contains('i') && '(\\\\$i)'.contains('42') && '(\\\\$i)'.contains('\\\\')
+
+  assert '(${i})' == '(42)'
+  assert '(\${i})'.contains('i') && !'(\${i})'.contains('42')
+  assert !'(\\${i})'.contains('i') && '(\\${i})'.contains('42') && '(\\${i})'.contains('\\')
+  assert '(\\\${i})'.contains('i') && !'(\\\${i})'.contains('42') && '(\\${i})'.contains('\\')
+  assert !'(\\\\${i})'.contains('i') && '(\\\\${i})'.contains('42') && '(\\\\${i})'.contains('\\\\')
+  assert i==42
+}
+
+fn test_implicit_str() {
+  i := 42
+  assert 'int $i' == 'int 42'
+  assert '$i' == '42'
+
+  check := '$i' == '42'
+  assert check
+
+  text := '$i' + '42'
+  assert text == '4242'
+}