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/graph.v

@@ -4,7 +4,8 @@ import gx
 import gg
 import time
 import glfw
-// import math
+import math
+import os
 
 const (
 	Size  = 700
@@ -16,6 +17,7 @@ struct Context {
 }
 
 fn main() {
+	os.clear()
 	glfw.init()
 	ctx:= &Context{ 
 		gg: gg.new_context(gg.Cfg {
@@ -40,11 +42,12 @@ fn main() {
 fn (ctx &Context) draw() {
 	ctx.gg.draw_line(0, Size / 2, Size, Size / 2) // x axis 
 	ctx.gg.draw_line(Size / 2, 0, Size / 2, Size) // y axis 
-	center := f64(Size / 2) 
+	center := f64(Size / 2)
+	mut y := 0.0
 	for x := -10.0; x <= 10.0; x += 0.002 {
-		y := x * x - 1 
-		//y := (x + 3) * (x + 3) - 1
-		//y := math.sqrt(30.0 - x * x)
+		y = x * x - 1 
+		//y = (x + 3) * (x + 3) - 1
+		//y = math.sqrt(30.0 - x * x)
 		ctx.gg.draw_rect(center + x * Scale, center - y * Scale, 1, 1, gx.Black) 
 		//ctx.gg.draw_rect(center + x * Scale, center + y * Scale, 1, 1, gx.Black) 
 	}

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)