lang: Check that set type matches actual expression

I forgot to include these two invariants which are occasionally
necessary, although in most cases they're necessary to prevent incorrect
code from getting past unification. In any case, they would have been
caught by the engine.

lang/parser.y

@@ -859,6 +859,7 @@ arg:
 	}
 ;
 bind:
+	// `$s = "hey"`
 	VAR_IDENTIFIER EQUALS expr
 	{
 		posLast(yylex, yyDollar) // our pos
@@ -867,6 +868,8 @@ bind:
 			Value: $3.expr,
 		}
 	}
+	// `$x bool = true`
+	// `$x int = if true { 42 } else { 13 }`
 |	VAR_IDENTIFIER type EQUALS expr
 	{
 		posLast(yylex, yyDollar) // our pos