statprof: add a couple of asserts to avoid storing unicodes

I don't feel strongly about this change, so it'd be fine to drop it
during review, but it was _extremely_ helpful in drafting the next
change.

Differential Revision: https://phab.mercurial-scm.org/D5209

mercurial/statprof.py

@@ -206,8 +206,10 b' class CodeSite(object):'
     __slots__ = (u'path', u'lineno', u'function', u'source')
 
     def __init__(self, path, lineno, function):
+        assert isinstance(path, bytes)
         self.path = path
         self.lineno = lineno
+        assert isinstance(function, bytes)
         self.function = function
         self.source = None