Oracle support.

The impetus for this was the fact that "SELECT foo FROM bar AS b" is
not valid SQL according to Oracle - it wants "SELECT foo FROM bar b"
instead (without the "AS").

The commit adds very basic support for the concept of a "dialect",
which I envision being a superset of the "quoting" support that is
already in `format`, but supporting all the quirks of particular
dialects (eventually).