Quite a few modules were missing copyright notices.
This commit adds copyright notices everywhere via haddock module
headers. The old license boilerplate comment is redundant with this and has
been removed.
Update copyright years to 2019.
Closes#4592.
* XML.toEntities: changed type to Text -> Text.
* Shared.tabFilter -- fixed so it strips out CRs as before.
* Modified writers to take Text.
* Updated tests, benchmarks, trypandoc.
[API change]
Closes#3731.