New version adds ~15ms to `ruby -e 'puts "yo"' time :)

Quite possibly broken for more complex cases, but it's a start.

Also, I renamed the hooks to have their event name ('which', 'exec',
'rehash') in their filename so they're easier to pick out in
`RBENV_DEBUG=1` output.

Where we're at:

    $ time ruby -e 'puts "yo"'
    yo

    real	0m0.173s
    user	0m0.097s
    sys	0m0.034s

Versus with all rbenv-gemset hooks removed:

    $ time ruby -e 'puts "yo"'
    yo

    real	0m0.135s
    user	0m0.078s
    sys	0m0.025s

So added ~40ms with the hooks (rehash isn't used in that run). Not bad.

While I'm sure it was there for a reason - I recall
there being weird and troublesome double
executions going on when I dug into the
rbenv-gemset stuff - it messes up subprocesses
that run one lock exe invoked from inside
another.