Introduce contract-level compile-time constants (#432)

* Allow compile-time constants as toplevel declarations

* Remove the test that fails on toplevel consts

* Warn when shadowing a constant

* Allow records to be used as compile time constants

* Allow data constructors in compile-time constants

* Disable some warnings for toplevel constants

Since variables and functions cannot be used in the definition of
a compile time constants, the following warnings are not going to be
reported:

* Used/Unused variable
* Used/Unused function

* Do not reverse constants declarations

* Add tests for all valid expressions

* Add test for accessing const from namespace

* Revert "Do not reverse constants declarations"

This reverts commit c4647fadacd134866e4be9c2ab4b0d54870a35fd.

* Add test for assigining constant to a constant

* Show empty map or record error when assigning to const

* Report all invalid constant expressions before fail

* Allow accessing records fields in toplevel consts

* Undo a mistake

* Add test for warning on const shadowing

* Show error message when using pattern matching for consts

* Remove unused error

* Ban toplevel constants in contract interfaces

* Varibles rename

* Change the error message for invalid_const_id

* Make constants public in namespaces and private in contracts

* Add a warning about unused constants in contracts

* Use ban_when_const for function applications

* Test for qualified access of constants in functions

* Add failing tests

* Add test for the unused const warning

* Update CHANGELOG

* Update all_syntax test file

* Treat expr and type inside bound as bound

* Allow typed ids to be used for constants

* List valid exprs in the error message for invalid exprs

* Fix tests

* Update the docs about constants

* Update syntax docs

* Check validity of const exprs in a separate functions

* Call both resolve_const and resolve_fun from resolve_var

test/contracts/all_syntax.aes

@@ -6,6 +6,7 @@
 namespace Ns =
   datatype d('a) = D | S(int) | M('a, list('a), int)
   private function fff() = 123
+  let const = 1
 
   stateful entrypoint
     f (1, x) = (_) => x
@@ -33,6 +34,8 @@ contract AllSyntax =
 
   type state = shakespeare(int)
 
+  let cc = "str"
+
   entrypoint init() = {
     johann = 1000,
     wolfgang = -10,