75b3f19b96 Move and rename TxOut default trait to a const called NULL (yancy)
Pull request description:
Create an associated constant `const TxOut::NULL` for consensus signing code and remove the default trait. Note I tried to deprecate the `default()` fn instead of just removing it but it doesn't seem to be possible. Also because `TxOut::NULL` is `const`, `ScriptBuf::new()` needed to be changed to `const fn`.
ACKs for top commit:
apoelstra:
ACK 75b3f19b96
Kixunil:
ACK 75b3f19b96
Tree-SHA512: ff61a2b1641a1ba32f183c27205af2d868dbc2eb47cf758c3d8315329d2c23e0b8a82ea0ab59d1de9add0d238f927165e2e4df014aab1ef066d74d4feda0700b