So "\u1234" creates a string with one character '\u1234' in it.
"\uu1234" however creates the string "\\u1234" with 6 characters in
it. The "\uu" sequence gets turned into "\\u". This is intentional, as
the code in the lexer says
/* A double-u quoted escape. Convert the "\u" or "\U" to "\",
* thereby shaving off a "u" or "U" from the escape
* sequence. */
Where does this weird syntax come from? Why is it desired?
