typing: add type hints to the `charencode` module Since this module is dynamically imported from either `mercurial.pure` or `mercurial.cext`, these hints aren't detected in `mercurial.encoding`, and need to be imported directly there during the type-checking phase. This keeps the runtime selection via the policy config in place, but allows pytype to see these as functions with proper signatures instead of just `Any`. We don't attempt to import the `mercurial.cext` version yet because there's no types stubs for that module, but this will get the ball rolling. I thought this would spill over into other modules from there, but the only two *.pyi files that changed were for `encoding` and `charencode`. Applying this to other dynamically selected modules will clean some things up in other files, so this is a start. I had originally redefined the functions in the type-checking block (like some of the `os.path` aliasing in `mercurial.util`), but this is better because we won't have another duplication of the definitions that may get out of date.