Applied an explicit "cause" to most if not all internally raised exceptions
that are raised from within an internal exception catch, to avoid
misleading stacktraces that suggest an error within the handling of an
exception.  While it would be preferable to suppress the internally caught
exception in the way that the ``__suppress_context__`` attribute would,
there does not as yet seem to be a way to do this without suppressing an
enclosing user constructed context, so for now it exposes the internally
caught exception as the cause so that full information about the context
of the error is maintained.

Fixes: #4849
Change-Id: I55a86b29023675d9e5e49bc7edc5a2dc0bcd4751

row.keys() is used by any use case that applies dict() to
a row.  Access of elements by string key is also a 2.0 deprecation
not 1.4 so for rudimental dict(row) support make sure that is all
a 2.0 thing.

Fixes current Alembic test suite.

Change-Id: I895496324133d615676cd76bc5f2c5f4a83e9131

This builds on cc718cccc0bf8a01abdf4068c7ea4f3 which moved
RowProxy to Row, allowing Row to be more like a named tuple.

- KeyedTuple in ORM is replaced with Row

- ResultSetMetaData broken out into "simple" and "cursor" versions
  for ORM and Core, as well as LegacyCursor version.

- Row now has _mapping attribute that supplies full mapping behavior.
Row and SimpleRow both have named tuple behavior otherwise.
LegacyRow has some mapping features on the tuple which emit
deprecation warnings (e.g. keys(), values(), etc).   the biggest
change for mapping->tuple is the behavior of __contains__ which
moves from testing of "key in row" to "value in row".

- ResultProxy breaks into ResultProxy and FutureResult (interim),
the latter has the newer APIs.   Made available to dialects
using execution options.

- internal reflection methods and most tests move off of implicit
Row mapping behavior and move to row._mapping, result.mappings()
method using future result

- a new strategy system for cursor handling replaces the various
subclasses of RowProxy

- some execution context adjustments. We will leave EC in but
refined things like get_result_proxy() and out parameter handling.
Dialects for 1.4 will need to adjust from get_result_proxy()
to get_result_cursor_strategy(), if they are using this method

- out parameter handling now accommodated by get_out_parameter_values()
EC method.   Oracle changes for this.  external dialect for
DB2 for example will also need to adjust for this.

- deprecate case_insensitive flag for engine / result, this
feature is not used

mapping-methods on Row are deprecated, and replaced with
Row._mapping.<meth>, including:

   row.keys()  -> use row._mapping.keys()
   row.items()  -> use row._mapping.items()
   row.values() -> use row._mapping.values()
   key in row  -> use key in row._mapping
   int in row  -> use int < len(row)

Fixes: #4710
Fixes: #4878
Change-Id: Ieb9085e9bcff564359095b754da9ae0af55679f0