Generalize signature of checkbounds(A, I...)

[jishnub]

This method checks if indices are within bounds, and throws an error if they aren't. Custom types such as Broadcasted, therefore, would only need to implement the bounds-checking logic, and may leave the error-throwing bit to Base. They may do this by specializing checkbounds(Bool, A::MyType, I...) instead of checkbounds(A, I...), and the fallback implementation of the latter would handle processing the result.

[mbauman]

I'm not thrilled about this, because without this restriction checkbounds(Bool, 1) becomes ambiguous in its meaning. Do you want the erroring method for an "indexing" expression like Bool[1]? Or do you want the non-erroring method for 1[]?

The docs are currently explicitly all about arrays. It might be nice to generalize it to all generic indexables (and we do already have string methods), but I think we'd need a different method dispatch design to accommodate that.

[mbauman]

I think it'd also be good to think about how this works in general with other indexable things. The name itself is quite array-ish — it implies that there are contiguous regions of valid indices. But indexing also works with sets of non-contiguous keys; like dicts and named tuples and such. Should folks expect this to work like an erroring haskey for them?