ByteVec/Str: special case ASCII iteration in next method for Str.

I used the sumchars example for simple benchmarking:

	function sumchars{S<:String}(a::Array{S})
	    t = Uint32(0)
	    @inbounds for s in a, c in s
	        t += Uint32(c)
	    end
	    return t
	end

With this change benchmarking with

	median([ @elapsed sumchars(words) for _ in 1:100 ])

I found the following performance characteristics:

	* Str vs. ASCIIString with    `@inbounds` – same speed
	* Str vs. ASCIIString without `@inbounds` – 14% slowdown
	* Str vs. UTF8String  with    `@inbounds` – 2.55x speedup
	* Str vs. UTF8String  without `@inbounds` – 75% speedup

This strikes me as good enough to replace both string types with a
single string type – the artist currently known as `Str`.