Replace Alex + Happy with Attoparsec.
One pass parser with tests to go with it.
Remove an irrelevant HTML test.

Trying to parse a string of the form ‘foo @bar baz’ fails on the old
parser (we get a Nothing due to no matching @) which means that the
string is taken literally and the newline hack existing on both parsers
(although I believe it is no longer necessary and is only there for
compatibility) doesn't get appended. In the new parser this
succeeds (as a regular DocString) and the newline hack will be accounted
for.

The options:
 - Add a newline to the ref file. This means that the test in turn will
 fail on the old parser and doesn't actually test anything worthwhile as
 this test is useless for the new parser.
 - Remove the @ in the string. The test loses its value for the old
 parser (that is, no parse error occurs in the deprecation message) but
 passes on both parsers and at the very least tests that the message
 does occur. As these tests are already conducted in other files, I
 believe that this whole test should be removed all together and this is
 what's being done here.
 - Detect when a string would fail on the old parser and attempt a 1:1
 mapping to the old behaviour. This would be hard to do, add complexity
 and outright not worth the effort. The lack of the newline in such
 place makes no actual visual difference and no old well-formed (and a
 lot of ill-formed) documentation will not see any visual change.