Define ranged integer types

[annevk]

Although tackling #87 is a pretty big endeavor, I wonder if we can start smaller, by defining things such as:

• A 16-bit unsigned integer is an integer in the range 0 to 65535, inclusive.

This could be shared across Web IDL, URL, and Declarative Web Push. And probably other specifications.

[domenic]

This seems reasonable to me. Maybe with an XXX box pointing to the problems of #87, like how we haven't defined mathematical operations on 16-bit unsigned integers...