Path description logic is duplicated across wallets

[mrnerdhair]

The logic wallet.<chain>DescribePath functions are duplicated in many different wallets. Some of the implementations have slight differences, but all are supposed to produce the same outputs -- with the notable exception of the Trezor and Ledger implementations, which produce different descriptions using their device-specific Ethereum path derivation schemes. Nevertheless, most of the logic is the same, and the specific Ethereum path derivation scheme could easily be provided as a parameter to a single implementation.

• Dry out the redundant implementations of *describePath into a single implementation in hdwallet-core.
• The central implementation should support several different Ethereum account derivation schemes:
  • Standard BIP44: m/44'/60'/n'/0/0 => Ethereum Account #n
  • Metamask-style (used by Trezor): m/44'/60'/0'/0/n => Ethereum Account #n
  • Old Ledger-style: m/44'/60'/0'/n => Ethereum Account #n
  • Modern Ledger-style: m/44'/60'/n'/0/0 => Ethereum Account #n, m/44'/60'/0'/n => Ethereum Account #n (Legacy)

Fixed by #420.