Add a function for materializing a container from its type

[tkf]

close #73

Possible APIs

1. into(T, xf, foldable) (as implemented ATM)

   • or into(xf, T, foldable)?

2. copy(xf, [T,] foldable):

   • copy(xf, T, foldable) = append!!(xf, Empty(T), foldable)
   • copy(xf, foldable) = copy(xf, constructorof(typeof(foldable)), foldable)
   • It makes sense as we already have copy!(xf, dest, src)?

3. map(xf, [T,] foldable)

Upstream:

• Add Empty: an empty container proxy BangBang.jl#33

[codecov-io]

Codecov Report

Merging #75 into master will decrease coverage by 0.28%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   84.17%   83.88%   -0.29%     
==========================================
  Files          17       17              
  Lines        1106     1111       +5     
==========================================
+ Hits          931      932       +1     
- Misses        175      179       +4

Impacted Files	Coverage Δ
src/Transducers.jl	100% <ø> (ø)	⬆️
src/processes.jl	88.11% <0%> (-3.2%)	⬇️
src/core.jl	68.38% <0%> (-0.65%)	⬇️
src/library.jl	93.5% <0%> (+0.51%)	⬆️

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[non-Jedi]

Very cool. Does this make Transducers work with TypedTables as is by magic (I see you added a dependency on Tables.jl to BangBang), or does JuliaData/TypedTables.jl#55 have to be resolved first?

EDIT: Nevermind, I just saw JuliaFolds/BangBang.jl#29 where the support for TypedTables was added. Thanks so much for doing the work for interop. It's highly appreciated.