Fix/adts

[EliasC]

This PR fixes some of the issues with the recent effort to add
abstract data types to Encore. The changes are a combination of
refactoring, bug fixes and feature additions. I have a few ideas
for other features we should support, but this PR should make ADTs
"stable enough" to be used. I will write up I have written an RFC about future
work. Excluding things like refactoring and improved errors for
parsing and typechecking, at least the following changes have been
made:

• data is a reserved keyword (which required parts of the
  standard library to be rewritten, meaning this is a breaking
  change).

• A case declaration can only be given a single type, and not,
  e.g. a composite capability.

• When a case has no fields, parentheses are optional:

  data Option
  case Some(x : int) : Option
  case None : Option
  

  This has the drawback that using a field-less constructor has a
  different syntax, since parentheses are required there:

  val n1 = None() : Option
  val n2 = None : Unit -> Option
  match Some(42) with
    case None => println("This is actually a catch all!")
    case Some(x) => println("This will not be run")
  end
  

  This is currently addressed by emitting a warning whenever a
  variable has the same name as an existing ADT case (e.g. on
  lines 2 and 4 above). In the future, I think parentheses should
  be optional in these cases as well.

• A case declaration must now use have a type declared using
  data (and not a class or trait).

• A case declaration can now override the methods declared in
  their corresponding data declaration. See test adt/expr.enc

• ADT values can now be nested in non-ADT structures, e.g. tuples
  and Maybes, and vice versa. See test adt/nestedMatch.enc.

• The contents of a polymorphic ADT can be extracted with a
  match. See test adt/polyAdt.enc.

Note that, with the exception of the added tests, this PR removes
more lines than it adds.