Merge branch 'stack-type-safety'

* stack-type-safety:
  Fix tests
  Initial stack typing

src/machine.rs

@@ -14,6 +14,7 @@ use Result;
 use heap::Heap;
 #[cfg(feature = "bonus:network")]
 use netstack::NetStack;
+use std::convert::TryInto;
 
 pub const MAGIC_HEADER: u32 = 0x1DEA_DFAD;
 
@@ -118,7 +119,7 @@ impl Machine {
     }
 
     pub fn get_tos(&mut self) -> Result<i32> {
-        self.cur_stack().top()
+        self.cur_stack().top()?.try_into()
     }
 
     pub fn cur_frame(&mut self) -> &mut Frame {