Freeze ActiveSupport::Duration#parts hash

[pixeltrix]

Durations are meant to be value objects and should not be mutated.

By doing this we can cache whether a duration is variable and give the + and - methods on ActiveSupport::Duration a performance boost. It also avoids two array allocations by avoiding the any? and the inline array definition. It's not going to make a big performance difference to apps but it makes the duration_of_variable_length? method 2.5 times faster:

Warming up --------------------------------------
             current   325.663k i/100ms
                 new   828.177k i/100ms
Calculating -------------------------------------
             current      3.274M (± 2.9%) i/s -     16.609M in   5.076744s
                 new      8.337M (± 1.9%) i/s -     42.237M in   5.067887s

Comparison:
                 new:  8337279.2 i/s
             current:  3274297.7 i/s - 2.55x  (± 0.00) slower

I've removed the parts= and value= methods since these should never be used on a duration.

I've also made parts return a copy of the frozen hash and added a _parts reader so that we can access the parts internally without paying the tax of the dup.