Added ZIP 32 support to the Ledger app by copying its implementation from the Zcash Ledger app. This could close #52. More specifically, the following changes have been made:

• Modified the Ledger app and its Rust library to support HD paths of length 3
• Added checks to ensure that the ZIP 32 paths have the correct prefix (32) and correct coin (877)
• Obtain ZIP 32 seed by doing a SLIP 10 derivation on the path m/44'/877'/0'/0'/0' from the root seed
• Copied and integrated ZIP 32 derivation code from Zcash Ledger app and applied MASP personalizations
• Changed the app to respond to viewing key request with an extended full viewing key (instead of a full viewing key)
• Modified the app to display the bech32 encoding of the full viewing key (like the Namada client)

The above changes were partly necessitated by the fact that the Namada client only supports the importation and usage of extended full viewing keys. Additional note: the behaviour of the app on non-hardened ZIP 32 paths is currently untested.