Lots 'o patches

I've just committed a bunch of patches, mostly to help with parsing and
type conversion. The quick summary:

1) The UNION construct will now try to coerce types across each UNION
clause. At the moment, the types are converted to match the _first_
select clause, rather than matching the "best" data type across all the
clauses. I can see arguments for either behavior, and I'm pretty sure
either behavior can be implemented. Since the first clause is a bit
"special" anyway (that is the one which can name output columns, for
example), it seemed that perhaps this was a good choice. Any comments??

2) The name data type will now transparently convert to and from other
string types. For example,

  SELECT USER || ' is me';

now works.

3) A regression test for UNIONs has been added. SQL92 string functions
are now included in the "strings" regression test. Other regression
tests have been updated, and all tests pass on my Linux/i686 box.

I'm planning on writing a section in the new docs discussing type
conversion and coercion, once the behavior becomes set for v6.4.

I think the new type conversion/coercion stuff is pretty solid, and I've
tested as much as I can think of wrt behavior. It can benefit from
testing by others to uncover any unanticipated problems, so let me know
what you find...

                      - Tom

Oh, requires a dump/reload to get the string conversions for the name
data type.