JSON Schema validator

[snej]

JSONSchema class parses a JSON Schema and validates Fleece Values against it. See header file for class documentation. You may want to look at the JSON Schema docs / tutorial.

Other Fleece API changes supporting this:

• Added slice::UTF8Length(), which counts the number of UTF-8 code points
• Added FLEvalJSONPointer(), a wrapper around fleece::impl::Path::evalJSONPointer()
• Added FLDictIterator_BeginSK(), exposing the existing ability to pass a SharedKeys to a Dict::iterator

Bug fixes:

• Fixed some limitations of fleece::impl::Path::evalJSONPointer() (it didn't handle escaped keys)
• Fixed a SharedKeys issue with empty-string keys

Optimizations:

• Optimized Dict lookup from a Dict::key (aka FLDictKey), mostly by avoiding locating the Dict's SharedKeys unless we have to, since that's very slow.