src/chrono/timezones.nim is not thread safe

[ajusa]

The timezones are stored in a plain var at https://github.com/treeform/chrono/blob/master/src/chrono/timezones.nim#L52-L54, which makes it not thread safe. Eg, calling formatIso(ts, timezone) gives the dreaded gcsafe compiler error.

In practice, the timezones are set once at the start of a program so it's unlikely for threads to actually have an issue, but it would be good to fix that in the library instead of asking consumers to annotate each usage with .cast(gcsafe)