Support asReadOnlyBuffer()

[niloc132]

Provide a readonly view into various buffer types. Unlike #8, this should make the entire buffer visible, but will not share byteOrder:

The content of the new buffer will be that of this buffer. Changes to this buffer's content will be visible in the new buffer; the new buffer itself, however, will be read-only and will not allow the shared content to be modified. The two buffers' position, limit, and mark values will be independent.

The new buffer's capacity, limit, position, and mark values will be identical to those of this buffer, and its byte order will be BIG_ENDIAN.