Template Haskell: support for view patterns

TH's Pat type doesn't include a constructor for view patterns, which would ceratinly be useful.

As an example, Reid Barton suggested using view patterns for creating a regular expression quasiquoter (rx) [1].

For example, to check if a string matches certain regular expresion:

example x = case x of
 [$rx|.*foo.*] -> "contains foo"
 _             -> "doesn't contain foo"

Using pattern views, the quasiquoter would expand to

example x = case x of
 ((=~".*foo.*")->True) -> "contains foo"
 _             -> "doesn't contain foo"

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