Convert OS to enum

compiler/main.v

@@ -33,13 +33,11 @@ const (
 	TmpPath            = vtmp_path()
 )
 
-// TODO V was re-written in V before enums were implemented. Lots of consts need to be replaced with
-// enums.
-const (
-	MAC     = 0
-	LINUX   = 1
-	WINDOWS = 2
-)
+enum Os {
+	MAC
+	LINUX
+	WINDOWS
+}
 
 enum Pass {
 	// A very short pass that only looks at imports in the begginning of each file
@@ -63,7 +61,7 @@ enum Pass {
 struct V {
 mut:
 	build_mode BuildMode
-	os         int // the OS to build for
+	os         Os // the OS to build for
 	nofmt      bool // disable vfmt
 	out_name_c string // name of the temporary C file
 	files      []string // all V files that need to be parsed and compiled

compiler/parser.v

@@ -41,7 +41,7 @@ mut:
 	cgen           *CGen
 	table          *Table
 	run            Pass // TODO rename `run` to `pass`
-	os             int
+	os             Os
 	pkg            string
 	inside_const   bool
 	expr_var       Var