Grammar: use text() instead of input.substring(peg$currPos, offset())
SpecGrammar.js: add a couple UUID tests
Grammar: remove unused uuid_URI rule, clean up uuid rule
Grammar: fix quoted_string_clean rule

In theory, the rule returns what's between the quotes in the input.

In practice, it was slicing off the first and last characters of the input,
then returning the result. This only worked if there wasn't any SWS before
the quoted string.

Now, it actually returns what's between the quotes.
Grammar: userinfo rule uses text().slice instead of input.substring

Note that this rule is (still) broken, in that data.user gets set to a string
containing both the user and the password.