Uses ICU's facilities to un-escape Unicode character sequences.
The following escape sequences are recognized:
\a, \b, \t, \n, \v, \?,
\e, \f, \r, \", \', \\,
\uXXXX (4 hex digits),
\UXXXXXXXX (8 hex digits),
\xXX (1-2 hex digits),
\ooo (1-3 octal digits),
\cX (control-X; X is masked with 0x1F).
For \xXX and \ooo, beware of non-valid UTF-8 byte sequences.
Note that some versions of R on Windows cannot handle
characters defined with \UXXXXXXXX.