Incredibly rough POC of a Process.spawn_private method

Essentially, we introduce the concept of a waitpid_private_handle, and a
VM-wide table of child PIDs -> such handles.

When spawning a process, it can optionally be bound to such a handle, in
which case that handle/pid combo is inserted into a global table.

Whenever Ruby waits on a child process, if there is an active handle for
that pid, we "steal" the status and stash it against the handle. We
pretend to the caller that there was no child process.

If the handle is used to perform the wait, however, we return the
(possibly previously-saved) status.

Essentially, if you don't have the handle, you can't waitpid the child.

internal/process.h

@@ -22,6 +22,7 @@
 #include "ruby/ruby.h"          /* for VALUE */
 #include "internal/imemo.h"     /* for RB_IMEMO_TMPBUF_PTR */
 #include "internal/warnings.h"  /* for COMPILER_WARNING_PUSH */
+#include "vm_core.h"
 
 #define RB_MAX_GROUPS (65536)
 
@@ -59,6 +60,7 @@ struct rb_execarg {
     unsigned exception_given : 1;
     struct rb_process_status *status;
     struct waitpid_state *waitpid_state; /* for async process management */
+    struct waitpid_private_handle *waitpid_private_handle;
     rb_pid_t pgroup_pgid; /* asis(-1), new pgroup(0), specified pgroup (0<V). */
     VALUE rlimit_limits; /* Qfalse or [[rtype, softlim, hardlim], ...] */
     mode_t umask_mask;
@@ -76,6 +78,7 @@ struct rb_execarg {
 
 /* process.c */
 rb_pid_t rb_call_proc__fork(void);
+void waitpid_private_handle_clear_table(rb_vm_t *vm);
 void rb_last_status_clear(void);
 static inline char **ARGVSTR2ARGV(VALUE argv_str);
 static inline size_t ARGVSTR2ARGC(VALUE argv_str);