Fix the R6RS exact-integer-sqrt and import into core guile

Fix the R6RS exact-integer-sqrt and import into core guile

* libguile/numbers.c (scm_exact_integer_sqrt): New C procedure to
  compute exact integer square root and remainder.
  (scm_i_exact_integer_sqrt): New Scheme procedure `exact-integer-sqrt'
  from the R6RS, imported into core guile.

* libguile/numbers.h: Add prototypes.

* module/rnrs/base.scm: Remove broken stub implementation, which would
  fail badly when applied to large integers.

* doc/ref/api-data.texi: Add documentation.

* doc/ref/r6rs.texi: Change documentation for `exact-integer-sqrt' to a
  stub that xrefs the core docs, as is done for other operations
  available in core.

* test-suite/tests/numbers.test: Add tests.

* NEWS: Add news entries.