Add a BroadcastStyle for AbstractFill

[jishnub]

Fixes #40
Fixes #58
Fixes #143
Fixes #145
Fixes #188

After this,

julia> Broadcast.BroadcastStyle(typeof(Fill(2,3)))
FillArrays.FillStyle{1}()

julia> Broadcast.BroadcastStyle(typeof(Zeros(2,3)))
FillArrays.ZerosStyle{2}()

A special ZerosStyle is necessary to properly handle broadcasting with structured matrix types from LinearAlgebra.

Often, we will have AbstractFill return types returned now in broadcasting operations, and --- where possible --- ones whose values are statically known:

julia> exp.(Zeros(4))
4-element Ones{Float64}

julia> (x -> x^2).(Zeros(4))
4-element Zeros{Float64}

julia> .!(Trues(4))
4-element Zeros{Bool}

This does not change the fact that broadcasting over a Fill is evaluated before others in a fused broadcast:

julia> ones(1,5) .+ (ones(1) .+ (_ -> rand()).(Fill("vec", 2)))
2×5 Matrix{Float64}:
 2.76296  2.76296  2.76296  2.76296  2.76296
 2.76296  2.76296  2.76296  2.76296  2.76296

This is achieved by recursively walking through a Broadcasted object, and evaluating the components that produce AbstractFills eagerly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (7b96cc2) to head (0984e80).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #385   +/-   ##
=======================================
  Coverage   99.91%   99.92%           
=======================================
  Files           9        9           
  Lines        1238     1291   +53     
=======================================
+ Hits         1237     1290   +53     
  Misses          1        1           

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[putianyi889]

This could also fix #394