Refactor input parsing in v0 API

Previously the server was using the Rego parse to conveniently parse
inputs into ast.Value. For large inputs this was taking unnecessarily
long (e.g., 60s for 500KB of JSON.)

server/server.go

@@ -1464,33 +1464,26 @@ func getExplain(p []string, zero types.ExplainModeV1) types.ExplainModeV1 {
 }
 
 func readInputV0(r io.ReadCloser) (ast.Value, error) {
-
 	bs, err := ioutil.ReadAll(r)
-
 	if err != nil {
 		return nil, err
 	}
-
-	s := strings.TrimSpace(string(bs))
-	if len(s) == 0 {
+	bs = bytes.TrimSpace(bs)
+	if len(bs) == 0 {
 		return nil, nil
 	}
-
-	term, err := ast.ParseTerm(s)
-	if err != nil {
+	var x interface{}
+	if err := util.UnmarshalJSON(bs, &x); err != nil {
 		return nil, err
 	}
-
-	return term.Value, nil
+	return ast.InterfaceToValue(x)
 }
 
 func readInputGetV1(str string) (ast.Value, error) {
 	var input interface{}
-
 	if err := util.UnmarshalJSON([]byte(str), &input); err != nil {
 		return nil, errors.Wrapf(err, "parameter contains malformed input document")
 	}
-
 	return ast.InterfaceToValue(input)
 }