cue: add unary constraints

[mpvl]

For instance:

a: >5         // This field must be greater than 5
b: !=null     // This field must not be null

c: >3.0 & <10  // (3.0, 10.0) open ended range on both sides

Supported operators:

< > >= <= != ==

These would be defined on the same types as their binary equivalents. Semantically such expressions would define disjunctions of possible values, analogous to ranges.

Introducing these would solve various limitations that are currently present. Such as defining open ended ranges for floats or requiring that a value should not be of a specific type.

[mpvl]

Implemented. Ranges are removed.

[cueckoo]

This issue has been migrated to cue-lang/cue#11.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.