Server URI label parsers are limited to 21 items

[david-perez]

We code-generate Nom parsers to parse URI request paths. The number of labels that can appear is limited to 21, because of tuple.

If someone models a route with more than 21 labels, we fail at codegen time.

This is just a tracking issue, we're not going to tackle this any time soon; 21 labels should be more than enough for almost all service implementations.