Implement basic exception framework, and simple for loop.

py/nlr.h

@@ -0,0 +1,26 @@
+// non-local return
+// exception handling, basically a stack of setjmp/longjmp buffers
+
+#include <limits.h>
+
+#ifndef __WORDSIZE
+#error __WORDSIZE needs to be defined
+#endif
+
+typedef struct _nlr_buf_t nlr_buf_t;
+struct _nlr_buf_t {
+    // the entries here must all be machine word size
+    nlr_buf_t *prev;
+    void *ret_val;
+#if __WORDSIZE == 32
+    void *regs[6];
+#elif __WORDSIZE == 64
+    void *regs[8];
+#else
+#error Unsupported __WORDSIZE
+#endif
+};
+
+unsigned int nlr_push(nlr_buf_t *);
+void nlr_pop();
+void nlr_jump(void *val) __attribute__((noreturn));