Switch to our own TurboPFor encoder.

Switch to our own TurboPFor encoder.

This is much slower (plocate-build becomes ~6% slower or so),
but allows us to ditch the external TurboPFor dependency entirely,
and with it, the SSE4.1 demand. This should make us much more palatable
for most distributions.

The benchmark program is extended with some tests that all posting lists
in plocate.db round-trip properly through our encoder, which found a
lot of bugs during development.