Specialize Iterators functions

[jishnub]

We may specialize Iterators.rest (and iterators.drop/Iterators.take analogously) to return AbstractFills. This ensures that we hit optimized methods that are defined for AbstractFills, instead of generic methods for iterators. As an example, the following works after this:

julia> allequal(Fill(4, ∞))
true

julia> @btime allequal($(Ref(Fill(4, ∞)))[]);
  8.403 ns (0 allocations: 0 bytes)

This also changes the behavior of slurping assignment a, b... = Fill(3, 4). After this, b is an AbstractFill as well, whereas previously, this used to be a Vector. This is achieved by specializing Base.rest.

julia> a, b... = Fill(3, 4)
4-element Fill{Int64}, with entries equal to 3

julia> b
3-element Fill{Int64}, with entries equal to 3

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (6f61dc3) to head (aa3b3a3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #390   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files           8        8           
  Lines        1147     1155    +8     
=======================================
+ Hits         1146     1154    +8     
  Misses          1        1           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.