[rvolosatovs]

Add account, admin, network and block.get-block queries to
starstream:ledger, served over POST /rpc like the existing ones:

• account.get-balance(key: list<u8>) and account.get-nonce(key)
  return the balance and last publish nonce of the account identified by
  its raw Ed25519 public key.
• admin.get-nonce() returns the last nonce used by the admin key to
  fund accounts, which is tracked separately from its publish nonce.
• network.get-name() returns the network every envelope is bound to.
• block.get-block(height) returns the envelopes of the block at
  height, in order, or none if there is no such block. Heights match
  block.get-height: block 1 holds the first envelope, and height 0
  (genesis, served by genesis.get-outputs) is none.

The client gains get_account_balance, get_account_nonce,
get_admin_nonce, get_network and get_block, and the CLI gains
account balance, account nonce, admin nonce, block get and
network name. --nonce of contract publish and account fund is
now optional and defaults to the last account or admin nonce reported
by the ledger plus one, respectively.

The client no longer carries a network: fund, publish_contract and
transact take the network to bind the signed envelope to, and fetched
envelopes are no longer checked against it. Reading a block, contract or
transaction from a ledger thus works regardless of its network. The CLI
drops the global --network argument and declares it only on the
commands that sign envelopes: account fund, contract publish and
contract script call. It defaults to the network reported by the
ledger. contract script call rejects both --network and --key
together with --simulate.

The contract.get-envelope, contract.get-wasm,
transaction.get-envelope and transaction.get-transaction handlers
now clone the stored Bytes under the contracts or transactions
read lock and release it before encoding the result, and UTXO method
calls release the contracts read lock before merging the UTXO state
into the contract Wasm, so writers are not stalled by large contracts.

get-error is shared by all lookups: its invalid-digest variant is
renamed to malformed, which also covers keys that are not valid
Ed25519 public keys.

API break: the generated GetError::InvalidDigest is now
GetError::Malformed. ClientBuilder::network and DEFAULT_NETWORK
are removed, and Client::{fund, publish_contract, transact} gain a
network parameter.

[rvolosatovs]

See #268 (comment)