read-extended-symbol handles backslash better, including r6rs hex escapes

read-extended-symbol handles backslash better, including r6rs hex escapes

* libguile/read.c (scm_read_extended_symbol): Interpret '\' as an escape
  character.  Due to some historical oddities we have to support '\'
  before any character, but since we never emitted '\' in front of
  "normal" characters like 'x' we can interpret "\x..;" to be an R6RS
  hex escape.

* test-suite/tests/reader.test ("#{}#"): Add tests.