propagate first and last for OffsetRanges

[jishnub]

Propagate first and last to the parent for OffsetArray(::AbstractRange). On master step was propagated anyway as this is not defined for abstractarrays in general. The current change is an optimization as it gets around the need to index into the array to find the first element.

The first and last elements for ranges are often obtained by explicitly accessing fields, so I don't think that it's possible to obtain these for wrapper types by using traits.

I'm not sure if this provides an immediate real-world performance benefit, so perhaps we may leave this open for now. I had created this keeping something like JuliaLang/julia#41945 in mind. If iterate uses traits then we may require a rapid evaluation of first and last elements for OffsetRanges to be as performant as ranges.

[codecov]

Codecov Report

Merging #258 (9b9b7f0) into master (f3fad96) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #258   +/-   ##
=======================================
  Coverage   95.41%   95.41%           
=======================================
  Files           5        5           
  Lines         436      436           
=======================================
  Hits          416      416           
  Misses         20       20           

Impacted Files	Coverage Δ
src/OffsetArrays.jl	98.29% <100.00%> (ø)

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