Parse common English dates without the general parser - #16058

Summary

• date -f on ordinary lines such as Sat Sep 20 22:11:30 2014 was about 2.4× slower than GNU date. Fixes perf(date): Slower than GNU #14750 for that input.
• Those lines are now parsed directly. A release timing of 2000 of them was 0.33 ms versus 3.38 ms through parse_datetime.
• Any other shape still goes through the general parser. A unit test checks the direct parse against that parser, including a one-digit day and a trailing space.

Test plan

• cargo test -p uu_date --lib english_datetime
• date -f on the date list from perf(date): Slower than GNU #14750 matches GNU output and is closer to its runtime