Add support for custom functions.

This allows expressions like jet.Func("FOO", String("test")) to be
emitted as FOO($1).

internal/jet/func_expression.go

@@ -801,3 +801,8 @@ func newTimestampzFunc(name string, expressions ...Expression) *timestampzFunc {
 
 	return timestampzFunc
 }
+
+// Func can be used to call an custom or as of yet unsupported function in the database.
+func Func(name string, expressions ...Expression) Expression {
+	return newFunc(name, expressions, nil)
+}

internal/jet/func_expression_test.go

@@ -176,3 +176,7 @@ func TestTO_ASCII(t *testing.T) {
 	assertClauseSerialize(t, TO_ASCII(String("Karel")), `TO_ASCII($1)`, "Karel")
 	assertClauseSerialize(t, TO_ASCII(String("Karel")), `TO_ASCII($1)`, "Karel")
 }
+
+func TestFunc(t *testing.T) {
+	assertClauseSerialize(t, Func("FOO", String("test"), NULL, MAX(Int(1))), "FOO($1, NULL, MAX($2))", "test", int64(1))
+}