Rename methods to avoid ObjC KVC collisions. (#1699)

Note: Breaking API change on the Dictionary classes.

The numeric value classes were using "Value" in the naming, but this silently
collided with the KVC category on NSObject; meaning KVC code could break up a
keypath and call these selectors with the wrong types leading to crashes (even
though the code all would compile cleanly).

- Rename the methods to use the "type" instead of literal "Value".
- Update all the impls and tests.
- Enable the warning that will catch issues like this in the future.

Fixes https://github.com/google/protobuf/issues/1616