Add R6RS backslash string escape

Add R6RS backslash string escape

R6RS suggests that '\b' should be a string escape for the backslash
character.

* libguile/read.c (scm_read_string): parse backspace escape

* test-suite/tests/strings.test (R6RS backslash escapes): new test
  (Guile extensions backslash escapes): remove R6RS escapes from test.

* doc/ref/api-data.texi (Strings): document new string escape