Combined the bench_open and bench_seal benchmarks

Now we only have one bench_seal_open benchmark instead of two different ones.
This simplifies comparison of performance to the box_inplace branch
Merge branch 'master' into box_inplace
Combined the bench_open_inplace and bench_seal_inplace benchmarks

Also removed bench_seal_precomputed and bench_open_precomputed from
curve25519xsalsa20poly1305 since they don't really measure anything
interesting.