Integration with setjmp/longjmp.

Defining libpng bindings, with longjmp catching, is
now possible.

* autoload.c (ffi_set_entries): Add setjmp symbol, which is
a new macro in stdlib/ffi.tl.

* ffi.c (jmp_buf_s): New symbol variable.
(mk_jmp_buf, rt_setjmp, longjmp_wrap): New functions.
(ffi_init): Initialize jmp_buf_s. Register
sys:rt-setjmp and longjmp intrinsics.

* ffi.h (jmp_buf_s): Declared.

* stdlib/ffi.h (setjmp): New macro. Rather than introducing
a new special operator, we use a run-time support function
called sys:rt-setjmp, which takes functional arguments.

* unwind.[ch] (uw_snapshot, uw_restore): New functions.
The rt_setjmp function needs these to restore our unwind
frame stack into a sane state after catching a longjmp,
which bails without unwinding it, leaving the pointers
referring to frames that no longer exist.

* tests/017/setjmp.tl,
* tests/017/setjmp.expected: New files.

* txr.1: Documented.

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