💥 Add a pausable Contract Module

[pcaversaccio]

🕓 Changelog

This PR adds a pausable contract module which can be used to implement an emergency stop mechanism that can be triggered by an authorised account. Please note that this contract does not include an access control mechanism (e.g., ownable) for triggering the emergency stop. Such functionality must be implemented by the importing contract, for instance:

from snekmate.auth import ownable
initializes: ownable

from snekmate.utils import pausable
initializes: pausable

exports: ...

...

@external
def pause():
    ownable._check_owner()
    pausable._pause()

@external
def unpause():
    ownable._check_owner()
    pausable._unpause()

Additionally, the version pragmas in all Vyper source files have been updated to target the latest master version v0.4.1rc2, aligning with the release of Vyper's newest release candidate version v0.4.1rc1. Eventually, all submodules have been updated to their most recent commits.

🐶 Cute Animal Picture

[AlbertoCentonze]

Left a few comments with very minor stuff, awesome work as usual! Feel free to ignore :)