[ion] Add Ion::stackSafe

This allows asserting the stack is within bounds

ion/include/ion.h

@@ -43,6 +43,9 @@ uint32_t random();
 // Decompress data
 void decompress(const uint8_t * src, uint8_t * dst, int srcSize, int dstSize);
 
+// Tells wether the stack pointer is within acceptable bounds
+bool stackSafe();
+
 }
 
 #endif