refactor: Avoid copies in FlatSigningProvider Merge #25748

Merge will create several copies unconditionally:

• To initialize the args a, and b
• ret, which is the merge of the two args

So change the code to let the caller decide how many copies they need/want:

• a, and b must be explicitly moved or copied by the caller
• ret is no longer needed, as a can be used for it in place "for free"

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24149 (Signing support for Miniscript Descriptors by darosior)
• #22838 (descriptors: Be able to specify change and receiving in a single descriptor string by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Related is #16116. Could you also pull in the benchmark from that?

ACK fa00d8f84dca8ad9c5a3580e0f7a027f472cdc41

Silent merge conflict with master

Rebased and used @Empact's idea to transform this into a member function from https://github.com/bitcoin/bitcoin/pull/16116/files (thanks), which is a lot faster than an elided copy and std::move(). I've kept the argument as r-ref to allow stealing the pointers, which again should be faster than copying the entries individually.

looks good, ACK fa3f15f2

ACK fa3f15f2dd94ae597a66037f5928fe4e90fe099d

Code review ACK fa3f15f2dd94ae597a66037f5928fe4e90fe099d. Confirmed that all the places std::move was added the argument actually did seem safe to move from. Compiler enforces that temporary copies are explicitly created in non-move cases.