lang: parser: Lexer should allow in keyword as a variable name

We move it downwards to allow this case. Whether we want to allow this
long-term or not is to be decided.

lang/interpret_test/TestAstFunc2/lexer-parser0.txtar

@@ -0,0 +1,16 @@
+-- main.mcl --
+import "fmt"
+
+$map = 55
+$fn = func($in) { # in is a special keyword
+	13
+}
+test fmt.printf("%d", $fn(0)) {}
+func fn($in) { # in is a special keyword
+	42 + $map
+}
+test fmt.printf("%d", $fn(0)) {}
+test fmt.printf("%d", fn(0)) {}
+-- OUTPUT --
+Vertex: test[13]
+Vertex: test[97]

lang/parser/lexer.nex

@@ -139,11 +139,6 @@
 			lval.str = yylex.Text()
 			return NOT
 		}
-/in/		{
-			yylex.pos(lval) // our pos
-			lval.str = yylex.Text()
-			return IN
-		}
 /\->/		{
 			yylex.pos(lval) // our pos
 			lval.str = yylex.Text()
@@ -306,6 +301,11 @@
 			lval.str = yylex.Text()
 			return IDENTIFIER
 		}
+/in/		{
+			yylex.pos(lval) // our pos
+			lval.str = yylex.Text()
+			return IN
+		}
 /[A-Z]([a-z0-9_]*[a-z0-9]+)?/
 		{
 			yylex.pos(lval) // our pos