Change "Lexicographical Order" (and related) to "Unicode code point order".

[gkellogg]

This was raised against another spec in https://github.com/w3c/rdf-canon/issues/18, and a similar change should be made here and in Framing. Note that this shouldn't affect any actual test results.

The definition from rdf-canon is the following:

Unicode code point order This refers to determining the order of two Unicode strings (A and B), using Unicode Codepoint Collation, as defined in [XPATH-FUNCTIONS], which defines a total ordering of strings comparing code points.

Also defines other terms related to "lexical".

See https://github.com/w3c/json-ld-api/issues/552.

[gkellogg]

Summary: Clarify that "lexicographical order" means "code point order" and change some wording to reflect.

See #153.