This adds new fields 'pubkeys' and 'embedded' to the RPC's output, and improves the
documentation for previously added 'witness_version' and 'witness_program' fields.

Make CKeyStore automatically known about the redeemscripts necessary for P2SH-P2WPKH
(and due to the extra checks in IsMine, also P2WPKH) spending.

This introduces two command line flags (-addresstype and -changetype) which control
the type of addresses/outputs created by the GUI and RPCs. Certain RPCs allow
overriding these (`getnewaddress` and `getrawchangeaddress`). Supported types
are "legacy" (P2PKH and P2SH-multisig), "p2sh-segwit" (P2SH-P2WPKH and P2SH-P2WSH-multisig),
and "bech32" (P2WPKH and P2WSH-multisig).

A few utility functions are added to the wallet to construct different address type
and to add the necessary entries to the wallet file to be compatible with earlier
versions (see `CWallet::LearnRelatedScripts`, `GetDestinationForKey`,
`GetAllDestinationsForKey`, `CWallet::AddAndGetDestinationForScript`).

Improvements and cleanups by John Newbery