REF/BENCH: tslibs-specific parts of asvs
Motivation: asv runs take way too long.  One way to get around that is to make it easier to identify\* which benchmarks to run for a given PR.  To that extent, if we can identify benchmarks that don't rely on anything outside of tslibs\**, then a PR that doesn't touch that code can skip those benchmarks.  We can do the same thing for other low-dependency modules like _libs.algos.

\* Ideally in an automated way, but I'm not there yet.
\*\* Here and elsewhere, "tslibs" informally includes tseries.offsets and parts of tseries.frequences (until we can get all of that into the tslibs directory)

@TomAugspurger will moving benchmarks cause asv to start new time series, or will it successfully link up the new location to the old?