Linear types and pattern synonyms

Linear types cannot be used in pattern synonyms. This ticket is a placeholder to track this as a feature request. If you're interested, please speak up.

As a first step, this requires design. For example, we could support

id :: a %1 -> a
id x = x

nonlinear :: a -> a
nonlinear x = x

f1 (id -> x) = h x
f2 (nonlinear -> x) = h x
f3 (Unrestricted x) = h x

The function f1 is linear iff h is; the function f2 is never linear; the function f3 is always linear. Perhaps this could be done by annotating the result of a pattern synonym:

pattern P1 :: a %One -> a %One
pattern P2 :: a %Many -> a %Many
pattern P3 :: a %One -> a %Many

but I haven't checked the details. This also requires thinking about the type of matchers in Core.