bench: Construct CTxOut and COutPoint in a single expression

[alexanderwiederin]

Replaces field-by-field mutation of CTxOut and COutPoint in two bench files with brace initialisation, which requires the size_t conversions to be made explicit.

Noticed while looking at #35994, where switching the proposed fix-it to {} surfaces implicit narrowing conversions like these.

The constructed values are unchanged.

Note: Only the sites where a conversion is involved are included in this PR; the remaining field-by-field construction in bench/ would be covered by #35994's follow-ups.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36019.

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ACK	josibake

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[josibake]

ACK 5d53932

LGTM! Left a non-blocking readability suggestion.

[josibake]

I think slightly better is:

tx1.vout.reserve(number_outputs);

for (size_t i = 0; i < number_outputs; ++i) {
    tx1.vout.push_back(
        CTxOut{static_cast<CAmount>(i) * CENT, CScript{}}
    );
}

.. as I find it improves the readability and also keeps the type being constructed clearly visible. If performance were a concern you could do:

tx1.vout.reserve(number_outputs);

for (size_t i = 0; i < number_outputs; ++i) {
    tx1.vout.emplace_back(
        static_cast<CAmount>(i) * CENT, 
        CScript{}
    );
}

but considering this really hurts the readability I wouldn't suggest it here.

[josibake]

Same suggestion as above with:

tx2.reserve(tx1.vout.size());
for (size_t i = 0; i < tx2.vin.size(); i++) {
    tx2.push_back(
        COutPoint{parent_txid, static_cast<uint32_t>(i)}
    );
}