RFC: add pointer(s::Union(ByteString,UTF16String,UTF32String))

[stevengj]

This patch adds pointer(s) for s::Union(ByteString,UTF16String,UTF32String) and substrings thereof. Not only is this more convenient than convert(Ptr{Foo}, s) (because s knows the correct pointer type), but it also works for arbitrary substrings (whereas convert throws an error if the substring is not at the end).

The patch also changes the chars field of UTF32String to data for consistency with our other string types; this makes it easier to write methods that work on Union(ByteString,UTF16String,UTF32String).

Also, I changed the signature in pointer{T}(x::AbstractArray{T}, i::Int) to pointer{T}(x::AbstractArray{T}, i::Integer), since I couldn't see any reason to restrict the index to Int here.

[aviks]

+1. This is a much cleaner interface.

[stevengj]

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