Import R6RS bytevectors and I/O ports from Guile-R6RS-Libs 0.2.

Import R6RS bytevectors and I/O ports from Guile-R6RS-Libs 0.2.

* README: Document dependency on GNU libunistring.

* benchmark-suite/Makefile.am (SCM_BENCHMARKS): Add
  `benchmark/bytevectors.bm'.

* configure.in: Make sure we have libunistring; update $LIBS.

* libguile.h: Include "bytevectors.h" and "r6rs-ports.h".

* libguile/Makefile.am (libguile_la_SOURCES): Add `bytevectors.c' and
  `r6rs-ports.c'
  (DOT_X_FILES): Add `bytevectors.x' and `r6rs-ports.x'.
  (DOT_DOC_FILES): Add `bytevectors.doc' and `r6rs-ports.doc'.
  (noinst_HEADERS): Add `ieee-754.h'.
  (modinclude_HEADERS): Add `bytevectors.h' and `r6rs-ports.h'

* libguile/validate.h (SCM_VALIDATE_BYTEVECTOR): New macro.

* module/Makefile.am (SOURCES): Add $(RNRS_SOURCES).
  (RNRS_SOURCES): New variable.

* test-suite/Makefile.am (SCM_TESTS): Add `bytevectors.test' and
  `r6rs-ports.test'.