Apply underscore pattern

[ericnordelo]

Fixes #795

The proposed pattern is the following:

• Avoid underscore by default, except in the following cases:
• When the function name collides with a function name outside the InternalImpl (e.g. approve vs _approve).
• When the function is intended as private, which is not meant to be used outside the component (it can be used in presets but special care should be taken) (e.g: _approve_with_optional_event).

PR Checklist

• [x] Tests
• [x] Documentation
• [x] Added entry to CHANGELOG.md
• [ ] Tried the feature on a public network

[ericnordelo]

I agree and will update accordingly. Thanks @andrew-fleming. BTW, I forgot to mention that this PR also changes the order of some functions in the internal implementation and is prioritizing not-underscored-functions in the order.