Lisu tone letters

[Jules-Bertholet]

Unicode says of the Lisu tone letters:

The first four tone letters can be used in combination with the last two to represent certain
combination tones. [...] In monospaced fonts where all letters have the same advance width
(for example, one em), it is desirable to fit such a combination of tone letters into the
advance width of a simple tone letter.

Unfortunately, this requires a revert of #46, so we need to judge which is more valuable.

[Manishearth]

It would be nice if this list of things could make it to the spec upstream.