Expose public garbage collector API for C language

You can now do epic things like this:

    puts(_gc(xasprintf("%d", 123)));

The _gc() API is shorthand for _defer() which works like Go's keyword:

    const char *s = xasprintf("%d", 123);
    _defer(free, s);
    puts(s);

Be sure to always use -fno-omit-frame-pointer which makes code fast too.

Enjoy! See also #114

libc/runtime/gc.h

@@ -0,0 +1,30 @@
+#ifndef COSMOPOLITAN_LIBC_RUNTIME_GC_H_
+#define COSMOPOLITAN_LIBC_RUNTIME_GC_H_
+#include "libc/calls/calls.h"
+#include "libc/nexgen32e/stackframe.h"
+#include "libc/runtime/runtime.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+void *_gc(void *) hidden;
+void *_defer(void *, void *) hidden;
+void __defer(struct StackFrame *, void *, void *) hidden;
+void __deferer(struct StackFrame *, void *, void *) hidden;
+void _gclongjmp(jmp_buf, int) nothrow wontreturn;
+
+#if defined(__GNUC__) && !defined(__STRICT_ANSI__)
+#define _gc(THING) _defer((void *)_weakfree, (void *)(THING))
+#define _defer(FN, ARG)                                                \
+  ({                                                                   \
+    autotype(ARG) Arg = (ARG);                                         \
+    /* prevent weird opts like tail call */                            \
+    asm volatile("" : "+g"(Arg) : : "memory");                         \
+    __defer((struct StackFrame *)__builtin_frame_address(0), FN, Arg); \
+    asm volatile("" : "+g"(Arg) : : "memory");                         \
+    Arg;                                                               \
+  })
+#endif /* defined(__GNUC__) && !defined(__STRICT_ANSI__) */
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_RUNTIME_GC_H_ */