Update calling convention

Previously, the stack would look like this when calling a function:

TOP
| arg2     |
| arg1     |
| function |
BOTTOM

This order is now reversed, with the function coming first and then the
args:

TOP
| function |
| arg2     |
| arg1     |
BOTTOM

This is a little more friendly to the stack-based machine. It is
slightly less intuitive as far as order of operations goes, but having a
function's arguments depend on the state of the function itself is a
little suspect and easy to work around if you truly need that.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/inst.rs

@@ -25,8 +25,8 @@ pub enum Inst {
     /// Pops the top item off of the stack, discarding it.
     Pop,
 
-    /// Pops the top N function arguments off the stack, and attempt to call the
-    /// next stack item.
+    /// Pops the top item off of the stack to call, and then pops off the
+    /// following N arguments.
     Call(usize),
 
     /// Exits the current function stack frame and returns control to the