Add a synthetic benchmark to represent unrolled filters vs looping and switching

On my machine, the unrolled version is about 17% faster.

BenchmarkFilterSliceUnrolled        1000           2128300 ns/op
BenchmarkFilterSlice        1000           2903004 ns/op
Update a performance observation
Rename two benchmarks for consistency
Go back to reusing the facttable between test runs in the non-synthetic benchmarks

This makes them run a lot faster.