New SubArrays based on stagedfunctions
Convert to parametric syntax
Make SubArray constructors parametric
Fix problem in getindex for DArrays
Support Colon as an index
SubArrays: efficient linear indexing for appropriate types

This incorporates important innovations of ArrayViews
sub(::SubArray,...): final index doesn't trigger linear indexing
sub always converts interior Int->UnitRange
SubArrays: add lots of tests
Add devdocs for SubArrays
SubArrays: fix first_index bottleneck in construction
Merge pull request #8501 from JuliaLang/teh/subarray

New array views based on stagedfunctions