Calls to `verbosity()` are slow

[Joseph Parker]

Calls to verbosity() in benchmarks can take ~50% of runtime, because it repeatedly calls get_environment_variable to get the GK_VERBOSITY environment variable, rather than just storing it.

This probably only affects benchmark; I don't think verbosity() is called heavily in gs2 itself.

[Joseph Parker]

Store verbosity value instead of recalling get_environment_variable

Addresses #7

→ <<cset f03c37f97329>>