be67831 Make DummySignatureCreator a singleton (Pieter Wuille)
190b8d2 Make BaseSignatureCreator a pure interface (Pieter Wuille)
Pull request description:
* Removes the `m_provider` field from `BaseSignatureCreator`. Instead both a `SigningProvider` (which provides keys and scripts) and a `BaseSignatureCreator` (which implements the transaction-specific (or other) signing logic) are passed into and down in `ProduceSignature`, making the two concepts orthogonal.
* Makes `BaseSignatureCreator` a pure interface without constructor, making it easier to implement new derivations of it (for example for message signing).
* As `DummySignatureCreator` now becomes a stateless object, turn it into a singleton `DUMMY_SIGNATURE_CREATOR`.
Tree-SHA512: 5f1f4512e4ea7d02a31df7b9ede55008efa716c5b74a2630ca1c2fc6599584d8bf5f5641487266127f4b3788033803539fbd22b03ef1219c83c10da2d3da3dcd
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