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.