Fix: Generate pattern signatures for constructors exported as patterns (#663)

* Fix pretty-printing of pattern signatures

Pattern synonyms can have up to two contexts, both having a
different semantic meaning: The first holds the constraints
required to perform the matching, the second contains the
constraints provided by a successful pattern match. When the
first context is empty but the second is not it is necessary
to render the first, empty context.

* Generate pattern synonym signatures for ctors exported as patterns

This fixes #653.

* Simplify extractPatternSyn

It is not necessary to generate the simplest type signature since
it will be simplified when pretty-printed.

* Add changelog entries for PR #663

* Fix extractPatternSyn error message

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