Introduce Token Map: An optimized static translation class.

This patch introduces a new class: `WP_Token_Map`, designed for efficient
lookup and translation of static mappings between string keys or tokens, and
string replacements (for example, HTML character references).

The Token Map imposes certain restrictions on the byte length of the lookup
tokens and their replacements, but is a highly-optimized data structure for
mappings with a very high number of tokens.

Developed in #5373
Discussed in https://core.trac.wordpress.org/ticket/60698

Fixes #60698.
Props: dmsnell, gziolo, jonsurrell, jorbin.


git-svn-id: https://develop.svn.wordpress.org/trunk@58188 602fd350-edb4-49c9-b593-d223f7449a82