Every mail row formatted its date through `toLocaleTimeString` and
`toLocaleDateString`, which build a new Intl formatter on each call; a
row from the last week built two, on every render. On Hermes that goes
through ICU over JNI and is the costly part of formatting a date.
`formatListDate` now keeps one `Intl.DateTimeFormat` per locale and
format in a module-level map. The output is unchanged, since those
methods format exactly as `Intl.DateTimeFormat` with the same options;
a test checks the two against each other across locales, formats and
ages. The cache starts over when the device's UTC offset changes, so a
new time zone still shows as it did when every call picked it up.
Linus Rath