Support for min/maxLength on object property names

[frontierpsycho]

(resubmitting from here)

It might be useful to be able to set limits on property names when defining them with patternProperties.

For example, I have an object which should have arbitrary key-value pairs, and I want to constrain the keys to be at most 64 characters. For now, I've resorted to this:

"additionalProperties": false,
"patternProperties": {
   "^\\S{1,64}$" : { ... }
}

(resubmitting this, from [here|https://github.com/json-schema/json-schema/issues/193#issuecomment-255453173])

However, the error message generated is very confusing. For example, if I send this object:

{ "SIXTYFOURCHARACTERSYOUSAYWELLALRIGHTTHENLETSNOTEXCEEDTHATSHALLWEIMEANWHONEEDSMORETHANSIXTYFOURCHARACTERSANYWAY":"value"}

I get the following error, which is clear to someone familiar with JSON-schema and the additional properties keyword, but not to everyone else:

SIXTYFOURCHARACTERSYOUSAYWELLALRIGHTTHENLETSNOTEXCEEDTHATSHALLWEIMEANWHONEEDSMORETHANSIXTYFOURCHARACTERSANYWAY: Additional properties not allowed

I suggest support for something like:

"patternProperties": {
  "^\\S*$": {
    "maxLength": 64,
    ...
  }
}

With the corresponding error message:

SIXTYFOURCHARACTERSYOUSAYWELLALRIGHTTHENLETSNOTEXCEEDTHATSHALLWEIMEANWHONEEDSMORETHANSIXTYFOURCHARACTERSANYWAY: Property name is too long (110 chars), maximum 64

[epoberezkin]

@frontierpsycho This is covered by propertyNames proposal that is more generic: #70

[frontierpsycho]

Indeed it is.