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

lang/structs.go

@@ -5255,6 +5255,15 @@ func (obj *ExprVar) Unify() ([]interfaces.Invariant, error) {
 		return nil, fmt.Errorf("var `%s` does not exist in this scope", obj.Name)
 	}
 
+	// if this was set explicitly by the parser
+	if obj.typ != nil {
+		invar := &unification.EqualsInvariant{
+			Expr: obj,
+			Type: obj.typ,
+		}
+		invariants = append(invariants, invar)
+	}
+
 	// don't recurse because we already got this through the bind statement
 	//invars, err := expr.Unify()
 	//if err != nil {
@@ -5531,6 +5540,15 @@ func (obj *ExprIf) Type() (*types.Type, error) {
 func (obj *ExprIf) Unify() ([]interfaces.Invariant, error) {
 	var invariants []interfaces.Invariant
 
+	// if this was set explicitly by the parser
+	if obj.typ != nil {
+		invar := &unification.EqualsInvariant{
+			Expr: obj,
+			Type: obj.typ,
+		}
+		invariants = append(invariants, invar)
+	}
+
 	// conditional expression might have some children invariants to share
 	condition, err := obj.Condition.Unify()
 	if err != nil {