RFC/WIP: Conditional 1-D Generators

[quinnj]

This is based on the currently open PR #14848.

This adds the ability, in the 1-D case, to support conditional generators, otherwise known as "guards".

Ref #550.

Example

julia> t = (i for i in 1:10)
Base.Generator{##1#2,UnitRange{Int64},Base.##1#2}(#1,1:10,Base.#1)

julia> collect(t)
10-element Array{Int64,1}:
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10

# manually construct a Generator with a "guard" or condition
julia> t = Base.Generator(x->x, 1:10, x->x%2 == 0)
Base.Generator{##7#9,UnitRange{Int64},##8#10}(#7,1:10,#8)

julia> collect(t)
5-element Array{Int64,1}:
  2
  4
  6
  8
 10

This doesn't yet support the necessary syntax parsing of the form:

t = (i for i in 1:10 if i % 2 == 0)

The collect implementation is probably too naive (not sure if we could get away with not having to evaluate every element), but I'm also not up to speed on the machinery map uses for determining the result element type.

This PR is more just to show a proof of concept and invite discussion of any other issues we'd need to consider with adding this feature.

[JeffBezanson]

That was quick :)

I think the right way to do this is to wrap the iterator with a Filter iterator.

[tkelman]

I like the sound of composing Generators with a Filter iterator.

[quinnj]

There's an old Julian tradition that if you take a stab at something, it's a good way to encourage Jeff to do it 10x better & sooner. It was fun playing around with this, but a Filter sounds much better.