Increase MAX_BIP32_PATH_STEPS to 10

[bigspider]

In order to better support use cases with longer derivation paths, it would be better to support longer BIP-32 paths for derivations and pubkey export.

To avoid memory problems on Nano S, we have to ascertain that increasing the constant does not lead to an increase in memory usage in critical paths, and in the global variables for UI (affected by MAX_SERIALIZED_BIP32_PATH_LENGTH).
Moreover, double check that the various UX flows are still reasonable when the paths are longer.