c00afe8d52 Change MessageSignatureError to secp256k1::Error (Jamil Lambert, PhD)
a20d0bc4eb Deprecate `from_slice()` in sha256.rs (Jamil Lambert, PhD)
089043546f Deprecate `from_slice` methods in favor of arrays (Jamil Lambert, PhD)
Pull request description:
As brought up in issue #3102 support for Rust arrays is now much better so slice-accepting methods that require a fixed length can be replaced with a method that accepts an array.
`from_slice()` methods that require a fixed length have been deprecated and where needed a `from_byte_array()` method created that accepts an array.
There are still `from_slice` methods that rely on changes to external crates before they can be changed to arrays.
ACKs for top commit:
apoelstra:
ACK c00afe8d52 successfully ran local tests
tcharding:
ACK c00afe8d52
Kixunil:
ACK c00afe8d52
Tree-SHA512: c505b78d5ca57e7e1004df6761ac6760d5d9b63c93edc6ac1f9adf047bc67011883575f835b06f6d35d7f6c2b6a4c6c7f0a82a3f0e293bfb4ef58123b75d3809