⚡️ Simplify and speed up SEARCH response parsing

The `mailbox-data` `SEARCH` response parser was refactored, as part of
the work on `CONDSTORE` support.  This commit contains only the
refactoring, which (coincidentally) brings a significant speed
improvement:

```
invalid_search_response_multiple_result_with_trailing_space
                v0.4.6-2-g3ac9912:     70865.1 i/s
                            0.4.6:     49505.1 i/s - 1.43x  slower

invalid_search_response_single_result_with_trailing_space
                v0.4.6-2-g3ac9912:     74398.0 i/s
                            0.4.6:     67791.1 i/s - 1.10x  slower

rfc3501_7.2.5_SEARCH_response_example
                v0.4.6-2-g3ac9912:     65968.9 i/s
                            0.4.6:     53490.9 i/s - 1.23x  slower

search_response_multiple_seq_nums_returned
                v0.4.6-2-g3ac9912:     62777.4 i/s
                            0.4.6:     47852.3 i/s - 1.31x  slower

search_response_single_seq_nums_returned
                v0.4.6-2-g3ac9912:     79656.5 i/s
                            0.4.6:     69234.9 i/s - 1.15x  slower

search_response_with_condstore_modseq
                v0.4.6-2-g3ac9912:     55376.4 i/s
                            0.4.6:     39129.0 i/s - 1.42x  slower
```

Author: nevans

lib/net/imap/response_parser.rb

@@ -1452,34 +1452,15 @@ def acl_data
       #   mailbox-data        = obsolete-search-response / ...
       #   obsolete-search-response = "SEARCH" *(SP nz-number)
       def mailbox_data__search
-        token = match(T_ATOM)
-        name = token.value.upcase
-        token = lookahead
-        if token.symbol == T_SPACE
-          shift_token
-          data = []
-          while true
-            token = lookahead
-            case token.symbol
-            when T_CRLF
-              break
-            when T_SPACE
-              shift_token
-            when T_NUMBER
-              data.push(number)
-            when T_LPAR
-              # TODO: include the MODSEQ value in a response
-              shift_token
-              match(T_ATOM)
-              match(T_SPACE)
-              match(T_NUMBER)
-              match(T_RPAR)
-            end
-          end
-        else
-          data = []
+        name = label_in("SEARCH", "SORT")
+        data = []
+        while _ = SP? && nz_number? do data << _ end
+        if lpar?
+          label("MODSEQ"); SP!
+          mod_sequence_value
+          rpar
         end
-        return UntaggedResponse.new(name, data, @str)
+        UntaggedResponse.new(name, data, @str)
       end
       alias sort_data mailbox_data__search