Implement `TypeEngine.as_generic`

Implement `TypeEngine.as_generic`

Added :meth:`_types.TypeEngine.as_generic` to map dialect-specific types,
such as :class:`sqlalchemy.dialects.mysql.INTEGER`, with the "best match"
generic SQLAlchemy type, in this case :class:`_types.Integer`.  Pull
request courtesy Andrew Hannigan.

Abstract away how we check for "overridden methods" so it is more
clear what the intent is and that the methodology can be
independently tested.

Fixes: #5659
Closes: #5714
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5714
Pull-request-sha: 91afb9a0ba3bfa81a1ded80c025989213cf6e4eb

Change-Id: Ic54d6690ecc10dc69e6e72856d5620036cea472a