Define mergewith[!]

[tkf]

This PR adds mergewith! and mergewith (and also merge). It's kind of nice that we can use curried form like mergewith!(+) now. Some random example:

julia> mapfoldl(mergewith!(+), rand(1000) .* maxintfloat()) do x
           dictionary(digits(floor(Int, x))[end] => 1)
       end |> pairs |> collect |> sort!
9-element Array{Pair{Int64,Int64},1}:
 1 => 119
 2 => 126
 3 => 117
 4 => 142
 5 => 121
 6 => 119
 7 => 123
 8 => 114
 9 => 19

[codecov-io]

Codecov Report

Merging #18 into master will decrease coverage by 3.19%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   61.79%   58.59%   -3.20%     
==========================================
  Files          18       18              
  Lines        1039     1135      +96     
==========================================
+ Hits          642      665      +23     
- Misses        397      470      +73     

Impacted Files	Coverage Δ
src/Dictionaries.jl	100.00% <ø> (ø)
src/insertion.jl	41.21% <83.33%> (+0.91%)	⬆️
src/indexing.jl	45.83% <0.00%> (-15.28%)	⬇️
src/broadcast.jl	51.78% <0.00%> (-6.22%)	⬇️
src/filter.jl	55.55% <0.00%> (-5.43%)	⬇️
src/tokens.jl	22.95% <0.00%> (-4.51%)	⬇️
src/Dictionary.jl	73.17% <0.00%> (-3.76%)	⬇️
src/map.jl	27.02% <0.00%> (-3.28%)	⬇️
src/AbstractDictionary.jl	72.06% <0.00%> (-2.94%)	⬇️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9b22a25...1267422. Read the comment docs.

[andyferris]

Thanks @tkf - this is a good idea.

Maybe I'm a bit odd but I been avoiding Compat.jl for foundational packages, just to minimize the number of dependencies etc...

[tkf]

I'm OK with removing Compat dependency. Actually, Compat.mergewith[!] would work with Dictionaries.jl even without the explicit support because the definition I added in Compat.jl is pretty generic: https://github.com/JuliaLang/Compat.jl/blob/1bb5dc7b9c98b4b4f834bebcefa813725dee0d43/src/Compat.jl#L360-L366