Allow for changing TZ vairable and cache it for Local timezone

[esheppa]

This should solve #834.

As discussed, I've done some basic benchmarks, and it seems that this has minimal effect on the standard /etc/localtime case, while being around 3% more expensive in the TZ variable case. However this seems justified as this now allows for the TZ variable to change.

empty TZ variable, timezone is a symlink at /etc/localtime

Before (e0b16e9…):

bench_get_local_time    time:   [79.480 ns 79.692 ns 79.923 ns]

After (360478c…):

bench_get_local_time    time:   [79.232 ns 79.383 ns 79.556 ns]

with TZ variable

Before (e0b16e9…):

bench_get_local_time    time:   [77.322 ns 77.664 ns 78.002 ns]

After (360478c…):

bench_get_local_time    time:   [79.711 ns 79.921 ns 80.155 ns]

[djc]

Good idea to add a benchmark! I think these are good changes, asking for a bit more structure in the commit history so it becomes easier to infer the rationale for the constituent parts.