Use a bespoke data type to indicate fixity

This deals with what I imagine was an ancient TODO and makes it much
clearer what the argument actually does rather than having the user
chase down the comment.

(cherry picked from commit 91e2c21cfdaca7913dbfec17bdd7712c0c1ed732)
Strip a single leading space from bird tracks (#201)

This makes bird tracks in the form

> foo
> bar
> bat

parse as if they had been written as

>foo
>bar
>bat

ie. without the leading whitespace in front of every line.

Ideally we also want to look into how leading whitespace affects code
blocks written using the @ @ syntax, which are currently unaffected by
this patch.

(cherry picked from commit fc7fd1875d31dbfd37eaa058177e534b4fc6bc25)
Turn a source code comment into specs

(cherry picked from commit dfc006ad2b8f5cb27570ac02c77aa63c5c027cd9)
Update test case for lifted GADT type rendering

The parsing of these seems to have been fixed by GHC folk and it now
renders differently. IMHO it now renders in a better way so I'm updating
the test to reflect this.

(cherry picked from commit 49b2a05ee0d769568d97a091de93f2ce2512eef6)
Don't shadow ‘strip’.

-Wall complains

(cherry picked from commit 1944b94edca881d14e979d564719da6f196f8e63)
Make ImplicitParams render correctly (#260)

This introduces a new precedence level for single contexts (because
implicit param contexts always need parens around them, but other types
of contexts don't necessarily, even when alone)

(cherry picked from commit 14531f7838c5abd0ba2aaf5217a477194d7b1897)
Lower precedence of equality constraints

This drops them to the new precedence pREC_CTX, which makes single
eqaulity constraints show up as (a ~ b) => ty, in line with GHC's
rendering. Additional tests added to make sure other type operators
render as intended. Current behavior matches GHC

(cherry picked from commit 64850ca4f7dc2ca0fdb21d078d93cd636de5c87a)
Add RankNTypes test case to ImplicitParams.hs

This test actually tests what #260 originally reported - I omitted the
RankNTypes scenario from the original fix because I realized it's not
relevant to the underlying issue and indeed, this renders as intended
now. Still good to have more tests.

(cherry picked from commit 6ca276702d04c9183caa98d1848f6aa5b88a8755)
Fix rendering of Contents when links are present

Fixes Haddock Trac #267.

(cherry picked from commit 1bf686940b28394f5d169f297659cd4c66869ec1)
Fix wording in the docs

(cherry picked from commit daa0ae5ba1acdee031cd72fc6690fe60a6710c20)