Add IRI support

Add IRI support

When getting the address of a URL, use the current scheme as URL type,
if one exists. If no scheme is provided for a URL, the current context
specifies the scheme to use, so if we’re looking at a gemini page,
then the default type is "gemini" even if elpher-default-url-type is
"gopher".

When creating the display-string for a page from a URL, use a new
functino that creates an IRI instead. It decodes percent-escapes and
handles punycode in the host name. It also hides the password, if any,
since that's what RFC 3986 says we should do.

When getting the display-string of a Gemini link line without a text,
do the same: don't just use the URL, use the corresponding IRI,
decoding percent-escapes and handling punycode in the host name.

When getting the URL of a Gemini link line, do the reverse: if this is
an IRI instead of a URI, use punycode for the host; percent-escaping
is already handled by url-generic-parse-url.