strings handled as built-in struct, and added immutable keyword

lexer.v

@@ -5,6 +5,7 @@ import term
 enum TokenType as u8 {
 	kw_let
 	kw_const
+	kw_immutable
 	type
 	kw_if
 	kw_else
@@ -108,6 +109,7 @@ fn toktype_from_kw(kw string) TokenType {
 		'true', 'false' {.boolean}
 		'print' {.kw_print}
 		'class' {.kw_class}
+		'immutable' {.kw_immutable}
 		else {.unknown}
 	}
 }
@@ -127,7 +129,7 @@ fn is_real(str string) bool {
 
 fn is_keyword(str string) bool {
 	return [
-		"void", "int", "real", "bool", "string", "if", "else", "elif", "for", "break", "fn", "return", "let", "const", "true", "false", "print", "class"
+		"void", "int", "real", "bool", "string", "if", "else", "elif", "for", "break", "fn", "return", "let", "const", "true", "false", "print", "class", "immutable"
 	].contains(str)
 }