Fully copy index expressions

Fully copy index expressions

Fixed bug where the :meth:`.Table.tometadata` method would not properly
accommodate :class:`.Index` objects that didn't consist of simple
column expressions, such as indexes against a :func:`.text` construct,
indexes that used SQL expressions or :attr:`.func`, etc.   The routine
now copies expressions fully to a new :class:`.Index` object while
substituting all table-bound :class:`.Column` objects for those
of the target table.

Also refined the means by which tometadata() checks if an Index
or UniqueConstraint is generated by a column-level flag, by propagating
an attribute "_column_flag=True" to such indexes/constraints.

Change-Id: I7ef1b8ea42f9933357ae35f241a5ba9838bac35b
Fixes: #4147