Summary:
I tried reading the lexer and the 2010 report side-by-side. Althought I didn't
quite finish, here are some small discrepancies that I found.

This revision may be low priority for reviewers, but having these commits just
in my local repository does no good either.

Changes:
* $nl was defined, but not used anywhere
* formfeed is a newline character
* add \: to $ascsymbol
  For simplification reason, the colon (':') was added to the character
  set $ascsymbol in the 2010 report. Here we make the same change.
* introduce the macros `qvarid`, `qconid`, `qvarsym` and `qconsym`
* foreign is a Haskell keyword
* add/update comments

Test Plan: Harbormaster (is awesome)

Reviewers: simonmar, hvr, austin

Reviewed By: austin

Subscribers: hvr, simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D180