Add MutableLinkedList

[c-p-murphy]

I put this together in response to issue #443. This is my first real contribution julia, so feel free to share any advice and/or feedback about how I could improve this implementation.

[codecov]

Codecov Report

Merging #450 into master will decrease coverage by 0.59%.
The diff coverage is 81.45%.

@@            Coverage Diff            @@
##           master     #450     +/-   ##
=========================================
- Coverage   91.32%   90.73%   -0.6%     
=========================================
  Files          29       30      +1     
  Lines        2133     2255    +122     
=========================================
+ Hits         1948     2046     +98     
- Misses        185      209     +24

Impacted Files	Coverage Δ
src/DataStructures.jl	100% <ø> (ø)	⬆️
src/mutable_list.jl	81.45% <81.45%> (ø)
src/heaps/arrays_as_heaps.jl	91.17% <0%> (-2.95%)	⬇️
src/deque.jl	97.84% <0%> (-0.04%)	⬇️

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[oxinabox]

Thanks for doing this.

@ararslan, @kmsquire I am really busy right now.
could either of you take a look at this?

Cross-ref
#442 and #443

Generally, speaking I am kinda opposed to adding more DataStructures to Data Structures.jl.
as I still think we really need to be doing: #310

But with that said, #310 is a fair bit of work and I've failed at it once, and don't see myself having free time to run with it anytime soon.
As such it really shouldn't block other things going forward, I guess.

[ararslan]

I think this is a good home for a mutable linked list type until the package is split up as Lyndon noted, if that ever fully happens. I just have a few comments.

[ararslan]

This is looking good to me, though it'd be nice to have another set of eyes on it as well.

[kmsquire]

Overall, looks good to me. I think it would be nice to bring the other LinkedList up-to-date and make the interface look more like this one. But that doesn't have to happen with this PR.

[kmsquire]

Nice implementation! I think that's fine. I'm curious if there's a type instability in there common case where the function return type doesn't change?

[c-p-murphy]

Thanks 😄

Unfortunately, it looks like there is (I forgot to check). @code_warntype highlights these lines red:

julia> @code_warntype map(f, l)
Body::MutableLinkedList{_1} where _1
.
.
.
5 ── . . .
│    %9  = (Core.Compiler.return_type)(f, %8)::Type
│    %10 = (Core.apply_type)(DataStructures.MutableLinkedList, %9)::Type{MutableLinkedList{_1}} where _1
│    %11 = (%10)()::MutableLinkedList{_1} where _1
.
.
.

Not sure what the best way to fix it is though. Is there a recommended approach?

[kmsquire]

Not that I'm aware of. In this case I think we just live with it.

Thanks for the contribution!