Added pretty.parse tests for SourceSyntax.Type

Pretty printing types should be using hsep and not sep. Not doing this will break the tests... long lambdas will span newlines and fail to parse.

Added the test case to parse pretty printed Types

Added the arbitrary instance for SourceSyntax.Type

Note - we should really get some source syntax newtypes for value and type identifiers. Would be nicer to encapsulate some of the invariants.