ports: Add custom binary output ports.
* modules/rnrs/io/ports.scm: Export `make-custom-binary-output-port'.
* src/ports.c (custom_binary_output_port_type, make_cbop, cbop_write,
scm_r6rs_make_custom_binary_output_port,
initialize_custom_binary_input_ports): New.
(scm_init_r6rs_ports): Call `initialize_custom_binary_output_ports ()'.
* src/ports.h (scm_r6rs_make_custom_binary_output_port): New.
* tests/io-ports.test ("8.2.10 Output
ports")["make-custom-binary-output-port",
"make-custom-binary-output-port [partial writes]",
"make-custom-binary-output-port [full writes]"]: New tests.
* doc/api-r6rs.texi (R6RS Binary Output): Add
`make-custom-binary-output-port'.