feat: support privilege management

[dl239]

support the following syntax:

GRANT priv_type ON [object_type] priv_level TO user_or_role[, user_or_role] [WITH GRANT OPTION]

REVOKE [IF EXISTS] priv_type ON [object_type] priv_level FROM user_or_role[, user_or_role]

SHOW GRANTS;

[aceforeverd]

pg merge user and group into single role, user just role with LOGIN attribute. This have many advantages, ref https://stackoverflow.com/a/8499491

allow groups to have groups as members

the ACL code would be simplified

the GRANT/REVOKE syntax and the display format for ACL lists could be simplified, since there'd be no need for a syntactic marker as to whether a given name is a user or a group.

In some circumstances I could see it making sense to allow logging in directly as a group/role/whatchacallit

This would also solve the problem that information_schema views will show only owned objects

[makes it easier to] representing privileges granted to groups [since you'd simply reuse the role related code?]

At least simplification make sense, e.g the privilege list. Consider ?

[aceforeverd]

just concern about extensibility.

Those two word privileges do require parser update for every new privilege, or it better to replace e.g 'CREATE USER' to 'CREATEUSER'.

Anyway you can reserve some common parts of the two words privileges, as synonyms. MySQL dialect is ugly :(

[dl239]

combine the two words looks more strange. how to set as synonyms

[aceforeverd]

not parser side, but privilege impl. Just point it out here

[aceforeverd]

refactored in #57 @dl239