Use optLast instead of listToMaybe for sourceUrls/wikiUrls

This lets you override them using eg. cabal haddock --haddock-options,
which can come in handy if you want to use a different layout or URL for
your source code links than cabal-install generates.
Differentiate between TH splices (line-links) and regular names

This adds a new type of source code link, to a specific line rather than
a specific declaration/name - this is used to link to the location of a
TH splice that defines a certain name.

Rather hefty changes throughout and still one unresolved issue (the line
URLs aren't parsed from the third form of --read-interface which means
they're currently restricted to same-interface links). Not sure if
this issue is really worth all the hassle, especially since we could
just use line links in general.

This commit also contains some cleanup/clarification of the types in
Haddock.Backends.Xhtml.Decl and shortens some overlong lines in the
process. Notably, the Bool parameter was replaced by a Unicode type
synonym to help clarify its presence in type signatures.