The generic template operator overloading was causing more problems than it
solved.  Eg:

https://github.com/behdad/harfbuzz/pull/163
https://github.com/behdad/harfbuzz/issues/175

So, just use macros.

Fixes https://github.com/behdad/harfbuzz/issues/175
Fixes https://github.com/behdad/harfbuzz/pull/178