Add back type-specific searches

[mccanne]

This is probably a low priority item.

Wiith zng-beta, we are getting rid of the port type and creating an alias to uint16 in the zeekio module.

As we evolve zql, its type, and the colon operator, we should be able to say

:port=80

as a boolean predicate (meaning search in all fields of type port for the value 80 a la the field index querying model). and perhaps bring back our short-hand for this...

:80

For that matter, we should be able to do any typed searches, e.g.,

:string=hello

etc.

[philrz]

When this issue gets addressed, there's skipped Rug tests that can be re-enabled. See brimdata/rug#206.