Helper to serialise BIP32 path

[Sjors]

A function like wally_map_keypath_get_item_path returns a uint32 array of path components. It would be nice to have a helper function to turn this into a string.

E.g. m/0h/0h/1

The "m" prefix is nice to have.

There should be a flag to choose between h and ' for hardened items.