Improve the surface language

[florentc]

There is room for several features that would greatly improve the surface language of Pirouette.

• Nested line-folding: support for nested line-folded sections is not trivial but would make it possible to get rid of the { ; ; ; } syntax in case statements and introduce let and where local function declarations. For now, line folding support is one-level only and relies on Reader (Maybe Pos) where Pos is the column number where a line folded section began.

• Implicitly quantified type variables: isJust : Maybe a -> Bool would be equivalent to isJust : forall a . Maybe a -> Bool

• Implicit naming of type variables in function body: for isJust : forall a . Maybe a -> Bool, the body declaration isJust mx = would be equivalent to isJust @a mx (name clashes and name shadowing should be kept in mind when implementing this). This requires to refactor and enhance funTerm in Language.Pirouette.QuasiQuoter.Syntax.

• Pattern matching directly in multiple function bodies (consider this after fixing Translate case expressions to SystemF #142): the Param type should rely on Pattern lang instead of String. The main question remains how to represent this when there are several pattern-matched function parameters: either as desugared nested case expressions ExprCase or in an adapted FunDecl type. The parser is ready for such a change and multi-bodies function declarations thanks to line folding, Pattern lang, and Param. This requires to refactor and enhance funTerm in Language.Pirouette.QuasiQuoter.Syntax so that it deals with pattern parameters.