Fix fcollect to respect object identity instead of ==; document order

[ericphanson]

Fixes #16. I checked the test I added failed on master:

julia> m1 = [1, 2, 3]
3-element Vector{Int64}:
 1
 2
 3

julia> m2 = [1, 2, 3]
3-element Vector{Int64}:
 1
 2
 3

julia> m3 = Foo(m1, m2)
Foo([1, 2, 3], [1, 2, 3])

julia> fcollect(m3)
2-element Vector{Any}:
 Foo([1, 2, 3], [1, 2, 3])
 [1, 2, 3]

julia> @test all(fcollect(m3) .=== [m3, m1, m2])
Error During Test at REPL[8]:1
  Test threw exception
  Expression: all(fcollect(m3) .=== [m3, m1, m2])
  DimensionMismatch("arrays could not be broadcast to a common size; got a dimension with lengths 2 and 3")

[darsnack]

Looks correct to me!

[DhairyaLGandhi]

If we intend to preserve the order, its worth a mention in the docs.

[ericphanson]

Ok, I wrote up the order defined by the implementation in the docstring. We could also not say exactly what the order is but just say it won't change for any inputs in a non-breaking release-- would that be preferred?

[darsnack]

I think the current explanation is more precise and understandable.