RFC: Add `@view` macro

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!