Fold Token::{Keyword, Identifier, DoubleQuotedString} into one
Token::SQLWord, which has the necessary information (was it a
known keyword and/or was it quoted).
This lets the parser easily accept DoubleQuotedString (a quoted
identifier) everywhere it expects an Identifier in the same match
arm. (To complete support of quoted identifiers, or "delimited
identifiers" as the spec calls them, a TODO in parse_tablename()
ought to be addressed.)
As an aside, per <https://en.wikibooks.org/wiki/SQL_Dialects_Reference/Data_structure_definition/Delimited_identifiers>
sqlite seems to be the only one supporting 'identifier'
(which is rather hairy, since it can also be a string
literal), and `identifier` seems only to be supported by
MySQL. I didn't implement either one.
This also allows the use of `parse`/`expect_keyword` machinery
for non-reserved keywords: previously they relied on the keyword
being a Token::Keyword, which wasn't a Token::Identifier, and so
wasn't accepted as one.
Now whether a keyword can be used as an identifier can be decided
by the parser. (I didn't add a blacklist of "reserved" keywords,
so that any keyword which doesn't have a special meaning in the
parser could be used as an identifier. The list of keywords in
the dialect could be re-used for that purpose at a later stage.)
