improvement: to_offset avoids binary search if the timestamp is greatr than the biggest one

[1996fanrui]

The subsequent optimization of #103

The logic inside of Tzif#to_offset

binary_search_by_key find the index from transitions, when the timestamp is greater than all timestamp in transitions. It's not needed to binary search.

It always happens when some timezones had DST before, but DST is canceled for now.

Change

Compare the timestamp with the biggest timestamp in transitions. If timestamp > self.transitions.last().unwrap().timestamp, we can avoid the binary search.

Performance change

In biref, the performance of instant_to_civil_datetime_static-Asia/Shanghai is improved 2 times via cthe urrent PR and #104

The performance of instant_to_civil_datetime_static-Asia/Shanghai :

• 99.3±1.35ns : Without current PR and improvement: ReasonablePosixTimeZone#to_offset returns default value directly when dst is none #104
• 68.4±5.85ns : master with improvement: ReasonablePosixTimeZone#to_offset returns default value directly when dst is none #104
• 50.1±1.02ns : With current PR and improvement: ReasonablePosixTimeZone#to_offset returns default value directly when dst is none #104

# before this PR and https://github.com/BurntSushi/jiff/pull/104(master branch)
group                                                base/chrono-tzfile/                    base/chrono/                           base/jiff/
-----                                                -------------------                    ------------                           ----------
instant_to_civil_datetime_static-Asia/Shanghai       1.00     34.7±0.51ns        ? ?/sec    1.08     37.4±0.62ns        ? ?/sec    2.87     99.3±1.35ns        ? ?/sec

# with https://github.com/BurntSushi/jiff/pull/104
group                                                base/chrono-tzfile/                    base/chrono/                           base/jiff/
-----                                                -------------------                    ------------                           ----------
instant_to_civil_datetime_static-Asia/Shanghai       1.00     34.5±0.98ns        ? ?/sec    1.10     38.1±2.72ns        ? ?/sec    1.98     68.4±5.85ns        ? ?/sec


# https://github.com/BurntSushi/jiff/pull/104 with current PR
group                                                base/chrono-tzfile/                    base/chrono/                           base/jiff/
-----                                                -------------------                    ------------                           ----------
instant_to_civil_datetime_static-Asia/Shanghai       1.00     34.6±1.09ns        ? ?/sec    1.15     39.7±6.03ns        ? ?/sec    1.45     50.1±1.02ns        ? ?/sec

[BurntSushi]

Nice! I buy it!

[1996fanrui]

Thanks @BurntSushi for the quick review and merge!