Guile: add value-{rvalue-,}reference-value

The Guile API doesn't currently have an equivalent to the Python API's
Value.reference_value() or Value.rvalue_reference_value(). This commit
adds a procedure with equivalent semantics to the Guile API.

gdb/ChangeLog:

	* NEWS (Guile API): Note the addition of new procedures.
	* guile/scm-value.c (gdbscm_reference_value): Add helper function
	for reference value creation.
	(gdbscm_value_reference_value): Add implementation of
	value-reference-value procedure.
	(gdbscm_value_rvalue_reference_value): Add implementation of
	value-rvalue-reference-value procedure.
	(value_functions): Add value-reference-value procedure.  Add
	value-rvalue-reference-value procedure.

gdb/doc/ChangeLog:

	* guile.texi (Values From Inferior In Guile): Add documentation
	for value-reference-value.  Add documentation for
	value-rvalue-reference-value.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-value.exp (test_value_in_inferior): Add test for
	value-reference-value.  Add test for value-rvalue-reference-value.