support text() in Index that's already table bound

[Mike Bayer]

no reason this shouldn't be possible

Table('x', metadata, Column('x', Integer),
    Index('asdf', text("func.foo.bar(x)"))
)

[Mike Bayer]

• Liberalized the contract for :class:.Index a bit in that you can specify a :func:.text expression as the target; the index no longer needs to have a table-bound column present if the index is to be manually added to the table, either via inline declaration or via :meth:.Table.append_constraint. fixes #3028

→ <<cset c33d0378802a>>