Simplify permission model for smart accounts

[nkrishang]

Summary

When using the AccountPermissions permission model for smart wallets on testnet (via the SDK and dashboard), we used it in a naturally simpler way than what the contract allows. This PR simplifies the permission model to match the natural way in which it was actually used on testnets.

Currently, AccountPermissions provides role based permissions. You can create a bytes32 role and associate a set of permissions with that role. This same role can then be assigned to any number of addresses.

// pseudo code for storage layout
bytes32[] roles;
mapping(bytes32 => Permissions) rolePermissions;
mapping(address => bytes32) roleOfSigner;

In practice, we ended up assigning unique roles for each address that was granted permissions to use the smart account. No one role was used for many addresses at all. In this PR, we simplify AccountPermissions to only support setting unique permissions for each address being granted permissions to use the smart account. This simplifies the contract's data structures and overall API.

// pseudo code for storage layout
mapping(address => Permissions) permissionsOfSigner;