Revise setinputsizes approach

Revise setinputsizes approach

in order to support asyncpg as well as pg8000,
we need to revise setinputsizes to work for more cases as well
as adjust NativeForEmulated a bit to work more completely with
the INTERVAL datatype.

- put most of the setinputsizes work into the compiler where
the computation can be cached.

- support per-element setinputsizes for a tuple

- adjust TypeDecorator so that _unwrapped_dialect_impl
will honor a type that the dialect links to directly in
it's adaption mapping.    Decouble _unwrapped_dialect_impl
from TypeDecorator._gen_dialect_impl() which has a different
purpose.   This allows setinputsizes to do the right thing
with the INTERVAL datatype.

- test cases for Oracle with Variant continue to work

Change-Id: I9e1ea33aeca3b92b365daa4a356d778191070c03