The internals of strftime were converted to use a single formatter,
instead of creating and concatenating a bunch of small strings. This
showed ~3x improvement in the benches.
Also, since the formatted time may be going straight to a Writer, TmFmt
was introduced, and is returned from all formatting methods on Tm. This
allows the saving of another string allocation. Anyone wanting a String
can just call .to_string() on the returned value.
This runs validation prior to return the created `TmFmt`, catching errors before formatting happens. The specialized formats skip this validation, since we already know they are valid.
[breaking-change]