Prefix private constants with underscore

[xenoliss]

This PR prefixes private constants with _.

A few notes on the adopted style guide

We are most likely aligned with the Solidity style guide:

1. For contracts, all private and internal symbols should be prefixed with an _.
2. For libraries:
   1. All private symbols should also be prefixed with an _.
   2. All internal symbols should NOT be prefixed with an _. This is because we consider that the library internal symbols are part of its publicly exposed API. This seems to be a generally accepted convention, as library internals are often used instead of public ones to avoid performing an expensive additional DELEGATECALL.

[wbnns]

@xenoliss

Thanks for taking the time to open this PR! We've added this to our backlog, pending to be reviewed.