JsLIGO: simple interfaces for namespace contracts

Motivation and Context

This MR implements simple interfaces for contracts in JsLIGO.

Description

It's basically front-end additions to JsLIGO.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

There's a new interface keyword for defining interfaces to be used in contracts (as namespaces):

interface ID {
  type storage;
  type ret = [list<operation>, storage];
  @entry const increment : (k: int, s: storage) => ret;
  @entry const decrement : (k: int, s: storage) => ret;
  @entry const reset : (_u: unit, s: storage) => ret;
};

namespace IncDec implements ID {
  type storage = int;
  type ret = [list<operation>, storage];

  @entry
  const increment = (delta : int, store : storage) : ret =>
    [list([]), store + delta];

  @entry
  const decrement = (delta : int, store : storage) : ret =>
    [list([]), store - delta];

  @entry
  const reset = (_ : unit, _ : storage) : ret =>
    [list([]), 0];
};

Using implements, when defining a namespace, we can check that the namespace (contract) implements the interface. Type declarations in interfaces can be left without definition (e.g. storage above), and the implementation can choose the concrete instance for the type.

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).