propose emulated setinputsizes embedded in the compiler

Add a new system so that PostgreSQL and other dialects have a
reliable way to add casts to bound parameters in SQL statements,
replacing previous use of setinputsizes() for PG dialects.

rationale:

1. psycopg3 will be using the same SQLAlchemy-side "setinputsizes"
   as asyncpg, so we will be seeing a lot more of this

2. the full rendering that SQLAlchemy's compilation is performing
   is in the engine log as well as error messages.   Without this,
   we introduce three levels of SQL rendering, the compiler, the
   hidden "setinputsizes" in SQLAlchemy, and then whatever the DBAPI
   driver does.  With this new approach, users reporting bugs etc.
   will be less confused that there are as many as two separate
   layers of "hidden rendering"; SQLAlchemy's rendering is again
   fully transparent

3. calling upon a setinputsizes() method for every statement execution
   is expensive.  this way, the work is done behind the caching layer

4. for "fast insertmany()", I also want there to be a fast approach
   towards setinputsizes.  As it was, we were going to be taking
   a SQL INSERT with thousands of bound parameter placeholders and
   running a whole second pass on it to apply typecasts.    this way,
   we will at least be able to build the SQL string once without a huge
   second pass over the whole string

5. psycopg2 can use this same system for its ARRAY casts

6. the general need for PostgreSQL to have lots of type casts
   is now mostly in the base PostgreSQL dialect and works independently
   of a DBAPI being present.   dependence on DBAPI symbols that aren't
   complete / consistent / hashable is removed

I was originally going to try to build this into bind_expression(),
but it was revealed this worked poorly with custom bind_expression()
as well as empty sets.   the current impl also doesn't need to
run a second expression pass over the POSTCOMPILE sections, which
came out better than I originally thought it would.

Change-Id: I363e6d593d059add7bcc6d1f6c3f91dd2e683c0c

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