compiler: streamline main function handling

* compiler: streamline C main function generation

* fix most tests

* compiler: fix for 'go update()' in graph.v . More precise parser error messages.

* Fix temporarily examples/hot_reload/message.v by using os inside it (os.clear).

* Make graph.v easier to quickly modify by defining y outside the loop.

* Fix failure of /v/nv/compiler/tests/defer_test.v when run with 'v -g' (#line directive was not on its own line, but right after } ).

* Do not pass the os.args to tests, even if the tests import os (they are more stable when run in a controlled environment).

* fix declared and not used in the js backend.

* fix js main => main__main too.

examples/hot_reload/message.v

@@ -2,6 +2,7 @@
 // v -live message.v
 module main
 
+import os
 import time
 
 [live]
@@ -10,6 +11,7 @@ fn print_message() {
 }
 
 fn main() {
+	os.clear()
 	for {
 		print_message()
 		time.sleep_ms(500)