Add value-returning match expressions

internal/lang/ast.go

@@ -217,6 +217,19 @@ type MatchCase struct {
 	Body                  []Stmt
 }
 
+type MatchExpr struct {
+	Value Expr
+	Cases []MatchExprCase
+}
+
+func (MatchExpr) exprNode() {}
+
+type MatchExprCase struct {
+	EnumName, VariantName string
+	Bindings              []string
+	Value                 Expr
+}
+
 type SelectCase struct {
 	Source Expr
 	Body   []Stmt