bug#38239: hungry-eol-escapes reader option doesn't allow intraline whitespace

[Andy Wingo]

Guile supports the R7RS idiom whereby this:

  "foo\
      bar"

reads as

  "foobar"

In the R6RS, this syntax is specified as allowing intraline whitespace
(space, tab, and anything Unicode with category Zs) after the \ but
before the newline.  However in Guile we don't support this and signal
an error instead ("illegal character in escape sequence: #\space").

Probably we should relax this restriction and allow intraline whitespace
after the \ and before the newline.