Use strftime to convert DateTime to numeric

The native implementation of the seconds since the UNIX epoch in
strftime is significantly faster than our method.

Benchmark:
----------
require 'benchmark/ips'
require 'date'
require 'time'

date = DateTime.civil(1253,7,6,20,4,0)

Benchmark.ips do |x|
  x.report("strftime.to_i") { date.strftime('%s').to_i }
  x.report("ssue.to_i") { ((date - DateTime.civil(1970)) * 86400).to_i }

  x.report("strftime.to_f") { date.strftime('%s').to_f }
  x.report("ssue.to_f") { ((date - DateTime.civil(1970)) * 86400).to_f }
end

Output:
-------
Calculating -------------------------------------
       strftime.to_i     26480 i/100ms
           ssue.to_i     13818 i/100ms
       strftime.to_f     26561 i/100ms
           ssue.to_f     14479 i/100ms
-------------------------------------------------
       strftime.to_i   616937.3 (±2.4%) i/s - 3098160 in 5.024749s
           ssue.to_i   200108.8 (±6.9%) i/s -  994896 in 4.999278s
       strftime.to_f   553581.3 (±2.2%) i/s - 2788905 in 5.040397s
           ssue.to_f   204260.3 (±4.3%) i/s - 1028009 in 5.043072s

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