Add @inbands, analogous to @inbounds

[dlfivefifty]

Create an @inbands to replicate the behaviour of @inbounds for the case where a user knows that the operation is inside the bands. This way the performance of @inbands A[k,j] should be equivalent to

@inbounds A.data[A.u + k - j + 1, j]

[dlfivefifty]

It looks like @inbounds is hard coded into inference.jl, so it's not possible to replicate it's behaviour.

I think it's OK to use @inbounds to mean inside the band. @inbounds implicitly implies that the user knows something about the data storage, so if the user uses @inbounds incorrectly, it's their own fault.

[dlfivefifty]

On second thought, this is a bad idea, as Base assumes @inbounds works for all indexes:

function copy!{T,N}(dest::AbstractArray{T,N}, src::AbstractArray{T,N})
    @boundscheck checkbounds(dest, indices(src)...)
    for I in eachindex(linearindexing(src,dest), src)
        @inbounds dest[I] = src[I]
    end
    dest
end