This patch dramatically simplifies the fallback indexing algorithms for arrays
by standardizing the way in which the passed indices are converted `to_index`
and by rigorously defining what a supported index type is.

The very first thing that occurs within the fallback indexing methods is that
the passed indices are converted to a supported indexing type by
`to_indices(...)`. A supported index type is either `Int` or an `AbstractArray`
of indices. This means that it is at this point that both `Colon` and arrays of
booleans get converted to collections of indices (as specialized array types:
`Slice` and `LogicalIndex`, respectively). This *drastically* simplifies much
of the internal logic. Whereas we had previously encoded the special behaviors
of `:` indexing in about four places, this patch does it once and then stores
the result in a `Slice` AbstractVector. Similarly, `CartesianIndex` is now
converted within `to_indices`, as well.

In addition to simplifying the internal definitions, this patch also makes it
easier for both external packages and Base to add new behaviors in a systematic
fashion. Simple extensions would include things like `Not` indices and
[EndpointRanges.jl](https://github.com/JuliaArrays/EndpointRanges.jl).

Additionally, logical indexing is now represented by a specialized
`LogicalIndex` type that enables fast iteration over the boolean mask without
allocations. This allows index conversions to occur before checking bounds,
and should enable further optimizations for LinearSlow arrays.

 ### Breaking Behaviors:

* The stored type of indices in SubArrays change from `Colon` to `Slice`.

* Move non-inlined function boundary up one method in the nonscalar indexing call chain and simplify the setindex call chain.
* Avoid calling indices(A) when it is known to be not needed. It'd be nice if Julia could identify this, but this is a very simple workaround.

Because the abstract fallbacks now rely more upon dispatch, the effect of a type-instability in the indices is greater.

…ianIndices.

This is a little unfortunate that we no longer try to call the canonical method multiple times when indexing with CartesianIndex, but I think the standardization is worth it.

This reverts commit f15ad88.

redundant with typevar and throwing a warning.