RFC: Add `@view` macro

[lendle]

This pull request adds a @view macro, which translates A[a:b, c:d] syntax to view(A, a:b, c:d).

Any symbol("end")s found in square brackets (unless they occur in nested square brackets) are converted to either size(A, d) or, if only one indexing object, length(A). The special case of 1:end is replaced by : instead of 1:size(A, d). For example:

• @view(A[5:end-2]) => view(A, 5:length(A)-2)
• @view(A[5:end, 3:b]) => view(A, 5:size(A,1), 3:b)
• @view(A[1:end, 3:b]) => view(A, :, 3:b)
• @view(A[c[3:end]]) => view(A, :, c[3:end])

@view only works on named arrays. @view(rand(10,10)[1:5, :]) will throw an error, because I don't think you'd want a view of an array that wasn't a named object very often.

I'm making some assumptions that I'm not 100% sure of, so please let me know if they could cause an issue:

• symbol("end") can only occur in an expression or subexpression of an expression with head :ref
• It always makes sense to replace symbol("end") with either a call to size or length
• If there is a symbol("end") in a :ref expression that is a subexpression of another :ref, the symbol("end") applies to the innermost :ref expression.

I also have a bunch of tests but I imagine they wouldn't be considered good practice for testing macros, so any feedback on that would be appreciated!

[lindahua]

@lendle This is great to have!

I think more tests to ensure correctness in more cases would be good too.