Remove enumerable from AccessControl and add an AccessControlEnumerable extension

[Amxx]

Fixes #2139

EnumerableSet and EnumerableMap heavily rely on sstore & sload, which are very expensive operations. At the same time, the Enumerable aspect can be tracked offchain using tools like thegraph.

Developer should have the choice between having this enumerable functionality (and pay the gas price), or drop it if onchain access to these information is sufficient.

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

Before making this decision we need to know what is the effect of having enumerability in AccessControl. What operations are affected and by how much.

For example, it seems that hasRole using EnumerableMap is only doing a single sload, which is the optimum.

Granting and revoking roles will be a lot more expensive with enumerability but these operations are not done often so it may be okay if they are not optimized for that.

Lastly, we also need to know the effect on contract size.

[Amxx]

Costs (from eth-gas-reporter):

Version	deploy mock	grant role	revoke role
master	580,730	84,860	24,827
non-enumerable	376,083	46,762	21,027

part of the saving for the deployment is from the smalest bytecode, and part is from granting role to msg.sender in the constructor

[Amxx]

Latest commit fixes #2139

[frangio]

Looks good! I left an improvement for the docs.

We also need to add {{AccessControlEnumerable}} in access/README.adoc.