Proto: expand constants before listing entrypoints

Context

Fixes #1906 (closed)

The MR changes the external signature of parse_toplevel making it asynchronous. This is warranted since with the introduction of global constants typechecking the code requires storage access in general. Internal to the module, the implementation of parse_toplevel is unchanged - rather, at the end the file I've added a call to Global_constants.expand before calling the original implementation.

Manually testing the MR

Deploy a contract with a constant and then run

tezos-client rpc get chains/main/blocks/head/context/contracts/<contract address>/entrypoints

The tezt test should give clues on how to do this.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR