Fix minor ledger reported issues

[spalmer25]

Ledger report some small issues:

• They didn't manage to run our tests. Done in https://github.com/trilitech/ledger-app-tezos-baking/pull/108
• There are still silenced warnings Makefile: remove silenced warning
• _A non properly documented function can be found in src/to_string.c:344._ to_string: improve the copy_string documentation
• _A CodeQL identifies For loop variable changed in the body in src/to_string:279_ microtez_to_string: clarify the trailing 0s elimination
• A CodeQL identifies Long switch case in src/operation.c:355-491 Postponed for another MR: https://github.com/trilitech/ledger-app-tezos-baking/issues/109

[spalmer25]

Can we add a test to check microtez to string conversion with a fractional microtez value with large trailing zeros. A value with no trailing zeros like 19.999999 tez.

Done: Test: test the tz display