SourceLayout: protocol_t upgrade

SourceLayout: protocol_t upgrade

This begins the libanyp.la SourceLayout changes by moving the protocol_t
type code to stand-alone files inside its namespace.

On the most part there are no behaviour changes. The boundaries between
the two semi-related types of protocol and protocol scheme are now clear:

 * URLScheme is to be used where the protocol name is related to a URI
   scheme description.

 * AnyP::ProtocolType is to be used for other non-URI places requiring
   the protocol to be named or manipulated as a concept.

Textual representations of these two concepts differ and the output of
these two types likewise differs to accomodate. Abusing them will result
in visibly unusual output.