* values.c, values.h: new files. use a struct to contain multiple
values, similar to the previous Scheme-level implementation.
* Makefile.am: add values.c, values.h, values.x.
* continuations.c (continuation_apply): support R5RS multiple value
continuations.
* init.c: call scm_init_values.
* struct.h: define SCM_SET_STRUCT_PRINTER.