Standardise the return value of `to_time`

This commit standardises the return value of `to_time` to an instance
of `Time` in the local system timezone, matching the Ruby core and
standard library behavior.

The default form for `String#to_time` has been changed from :utc to
:local but research seems to suggest the latter is the more common form.

Also fix an edge condition with `String#to_time` where the string has
a timezone offset in it and the mode is :local. e.g:

  # Before:
  >> "2000-01-01 00:00:00 -0500".to_time(:local)
  => 2000-01-01 05:00:00 -0500

  # After:
  >> "2000-01-01 00:00:00 -0500".to_time(:local)
  => 2000-01-01 00:00:00 -0500

Closes #2453
Use `DateTime.parse` inside `String#to_datetime`

Use the standard library's `DateTime.parse` because it's marginally
faster and supports partial date/time strings.

Benchmark:
       user     system      total        real
old  3.980000   0.000000   3.980000 (  3.987606)
new  3.640000   0.010000   3.650000 (  3.641342)