Fix function type signature and sentinel in method table
Fix function type signature and sentinel in method table

This commit fixes two method table issues.

First, it fixes the type signature of the "stop" function, otherwise,
as far as I know, undefined behavior is invoked (and may or may not
cause weird things immediately).

Second, it fixes the sentinel of the method table, as described in [1]
and [2].

[1]
https://docs.python.org/2.7/extending/extending.html#the-module-s-method-table-and-initialization-function
[2]
https://docs.python.org/3/extending/extending.html#the-module-s-method-table-and-initialization-function

sumerc/yappi,sumerc/yappi