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

docs/sophia_features.md

@@ -573,6 +573,32 @@ contract C =
 
 A hole expression found in the example above will generate the error `` Found a hole of type `(int) => int` ``. This says that the compiler expects a function from `int` to `int` in place of the `???` placeholder.
 
+## Constants
+
+Constants in Sophia are contract-level bindings that can be used in either contracts or namespaces. The value of a constant can be a literal, another constant, or arithmetic operations applied to other constants. Lists, tuples, maps, and records can also be used to define a constant as long as their elements are also constants.
+
+The following visibility rules apply to constants:
+*  Constants defined inside a contract are private in that contract. Thus, cannot be accessed through instances of their defining contract.
+* Constants defined inside a namespace are public. Thus, can be used in other contracts or namespaces.
+* Constants cannot be defined inside a contract interface.
+
+When a constant is shadowed, it can be accessed using its qualified name:
+
+```
+contract C =
+  let c = 1
+  entrypoint f() =
+    let c = 2
+    c + C.c  // the result is 3
+```
+
+The name of the constant must be an id; therefore, no pattern matching is allowed when defining a constant:
+
+```
+contract C
+  let x::y::_ = [1,2,3]  // this will result in an error
+```
+
 ## Arithmetic
 
 Sophia integers (`int`) are represented by arbitrary-sized signed words and support the following