Stage 3 of the faithful libyaml scanner port, continued. Translates fetch_tag, scan_tag, scan_tag_handle, scan_tag_uri, and scan_uri_escapes: verbatim (!<uri>), shorthand (!handle!suffix), and local (!suffix) tags, producing the handle and suffix for the parser to resolve. URI %-escapes are decoded by collecting the UTF-8 octets and decoding them as a unit. Unit-tested for a secondary-handle shorthand tag, a local tag, and a verbatim tag. Only directives (%YAML/%TAG) remain unported in the scanner. 317 tests, no regression.
