Reader option for R6RS hex escapes

Reader option for R6RS hex escapes

This adds a reader option 'r6rs-hex-escapes that modifies the
behavior of numeric escapes in characters and strings.  When enabled,
variable-length character hex escapes (#\xNNN) are allowed and become
the default output format for numerically-escaped characters.  Also,
string hex escapes switch to a semicolon terminated hex escape (\xNNNN;).

* libguile/print.c (PRINT_CHAR_ESCAPE): new macro
  (iprin1): use new macro PRINT_CHAR_ESCAPE

* libguile/private-options.h (SCM_R6RS_ESCAPES_P): new #define

* libguile/read.c (scm_read_opts): add new option r6rs-hex-escapes
  (SCM_READ_HEX_ESCAPE): modify to take a terminator parameter
  (scm_read_string): parse R6RS hex string escapes
  (scm_read_character): parse R6RS hex character escapes

* test-suite/tests/chars.test (with-read-options): new procedure
  (R6RS hex escapes): new tests

* test-suite/tests/strings.test (with-read-options): new procedure
  (R6RS hex escapes): new tests