In functional tests it is a quite common scenario to generate fresh
elliptic curve keypairs, which is currently a bit cumbersome as it
involves multiple steps, e.g.:
privkey = ECKey()
privkey.generate()
privkey_wif = bytes_to_wif(privkey.get_bytes())
pubkey = privkey.get_pubkey().get_bytes()
Simplify this by providing a new `generate_keypair` helper function that
returns the private key either as `ECKey` object or as WIF-string
(depending on the boolean `wif` parameter) and the public key as
byte-string; these formats are what we mostly need (currently we don't
use `ECPubKey` objects from generated keypairs anywhere).
With this, most of the affected code blocks following the pattern above
can be replaced by one-liners, e.g.:
privkey, pubkey = generate_keypair(wif=True)
Note that after this commit, the only direct uses of `ECKey` remain in
situations where we want to set the private key explicitly, e.g. in
MiniWallet (test/functional/test_framework/wallet.py) or the test for
the signet miner script (test/functional/tool_signet_miner.py).
