For non-printable and certain special (well-known,
see also the R man page Quotes)
ASCII characters, the following
(also recognized in R) convention is used.
We get \a, \b, \t, \n, \v,
\f, \r, \", \', \\
or either \uXXXX (4 hex digits) or \UXXXXXXXX (8 hex digits)
otherwise.
As usual in stringi, any input string is converted to Unicode
before executing the escape process.