Uses ICU facilities to unescape Unicode character sequences.
The following ASCII standard escapes are recognized:
\a, \b, \t, \n, \v, \?,
\e,
\f, \r, \", \', \\.
Moreover, the function understands the following ones:
\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 UTF8 byte sequences.
Note that some versions of R on Windows cannot handle
characters defined with \UXXXXXXXX.
We are working on that.