Use BTC_CURVE in test/test_ec.py.
Use BTC_CURVE in node/transport.py
Refactor pubkey_to_pyelliptic.

* Tag key variables with number base.
* Avoid destructive updates, so that code flow is super-clear.
* DRY key processing.
* As principle, never output keys to stdout, unless necessary.
Factor pubkey processing.
DRY pubkey processing.
Refactor makePubCryptor, hexToPubkey, makePrivCryptor.

* Tag key variables with number base.
* Avoid destructive updates, so that code flow is super-clear.
* DRY key processing.
* Added "FILLME"; please fill :)
Abstract EC parametres.
Refactor pubkey_to_pyelliptic (part 2)

* Drop _pubkey256ToPubkeyHex.
* Drop unnecessary base changes.
* Factor hexToPubkey code.
Drop hexToPubkey. Use pubkey_to_pyelliptic.

* Convert resulting pyelliptic pubkey from hex to binary when used.