fix construct_params() for render_postcompile; add new API

fix construct_params() for render_postcompile; add new API

The :meth:`.SQLCompiler.construct_params` method, as well as the
:attr:`.SQLCompiler.params` accessor, will now return the
exact parameters that correspond to a compiled statement that used
the ``render_postcompile`` parameter to compile.   Previously,
the method returned a parameter structure that by itself didn't correspond
to either the original parameters or the expanded ones.

Passing a new dictionary of parameters to
:meth:`.SQLCompiler.construct_params` for a :class:`.SQLCompiler` that was
constructed with ``render_postcompile`` is now disallowed; instead, to make
a new SQL string and parameter set for an alternate set of parameters, a
new method :meth:`.SQLCompiler.construct_expanded_state` is added which
will produce a new expanded form for the given parameter set, using the
:class:`.ExpandedState` container which includes a new SQL statement
and new parameter dictionary, as well as a positional parameter tuple.

Fixes: #6114
Change-Id: I9874905bb90f86799b82b244d57369558b18fd93