Add Time#deconstruct_keys - #6594

I believe that Time being suitable for pattern-matching is a reasonable feature with many possible usages, which will increase usability of Time and would be a good show case for pattern matching.

Implementation decisions

UPD: As per Matz's decision, only #deconstruct_keys is implemented now.

Time#deconstruct_keys:

• chosen keys: [:year, :month, :day, :yday, :wday, :hour, :min, :sec, :subsec, :dst, :zone]
• I am open to discussing whether we should include other subsecond units (or any whatsoever)
• It might be useful (but too loose interface) to support mon as a synonym for month?.. But might be confusing if somebody will unpack the **rest
• day, not mday, seems most reasonable

Possible usages:

case t
in year: ...2022
  puts "too old"
in month: ..9
  puts "quarter 1-3"
in wday: 1..5, month:
  puts "working day in month #{month}"
end

if t in Time(wday: 3, day: ..7)
  puts "first Wednesday of the month"
end

Time#to_h: added on a "why not" basis :) As we already have "convert to hash" in the form of deconstruct_keys(nil), having a canonic form seems harmless. Open for discussion.
Time#deconstruct: returns time components in order [year, month, mday, hour, min, sec, subsec]