Enhancement: Request at least N bytes of space in buffers

A new codec is implemented by extending the process function, which gives you a number of input bytes to transcode, and a number of bytes to write the result to. Right now, there is, as far as I can see, no way of requesting at least N bytes of input/output space. This makes it a little annoying to implement blocked codexes.

E.g. in the following example:

julia> open("/tmp/file", "w") do file
           write(file, rand(UInt8, Int(1.5 * 1 << 17)))
       end
196608

julia> stream = NoopStream(open("/tmp/file"); bufsize=1<<18)
TranscodingStream{Noop,IOStream}(<mode=idle>)

julia> read(stream, UInt8); bytesavailable(stream)
131071

Here, even though the stream has 2^18 bytes of buffer, and the file is 1.5*2^17, only 2^17 bytes is read into the buffer at a time.

What assumptions can we make about the number of available bytes? Could we, somehow, force the codec to make at least N bytes available, unless we read the EOF?

Start by reading the codec process function contract and tracing the NoopStream example to understand how input and output buffer sizes are chosen. Determine the semantics needed for requesting at least N bytes, including EOF behavior; the work is done when that contract is agreed and the example can obtain the requested buffer space.