move to new index format

Merge of the experimental ``font-matching`` branch.
The branch constitutes the bulk of the changes that will eventually become version 2.4 (targeting next texlive).

Addresses:

* https://github.com/lualatex/luaotfload/issues/118
* https://github.com/lualatex/luaotfload/issues/108
* https://github.com/lualatex/luaotfload/issues/142