refactor: Reduce memory copying operations in bech32 encoding

[paplorinc]

Started optimizing the base conversions in TryParseHex, Base58 and IsSpace - this is the next step.

Part of this change was already merged in #30047, which made decoding ~26% faster.

Here I've reduced the memory reallocations and copying operations in bech32 encode, making it ~15% faster.

make && ./src/bench/bench_bitcoin --filter='Bech32Encode' --min-time=1000

Before:

|             ns/byte |              byte/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|               19.97 |       50,074,562.72 |    0.1% |      1.06 | `Bech32Encode`

After:

|             ns/byte |              byte/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|               17.33 |       57,687,668.20 |    0.1% |      1.10 | `Bech32Encode`

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ACK	josibake, sipa

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[josibake]

Concept ACK

I cherry-picked your last commit into #30047 to get rid of the hardcoded values inside ExpandHRP. Looking at the rest of your commits here, the rest of the changes look great and the benchmark numbers are a nice improvement. Will review more thoroughly this week!

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_{Debug: https://github.com/bitcoin/bitcoin/runs/25832897694}

[paplorinc]

@josibake, now that the cherry-pick was merged, I've rebased and re-measured - the important part of this change was already included in the other PR, the remaining optimizations here are smaller, but also a lot simpler.

[josibake]

ACK 07f6417

Verified the benchmark locally. Aside from the speed improvements, this also improves the readability of the code.

[sipa]

utACK 07f6417

Making the code more readable is a good reason to make this change, but this code was specifically written to prioritize simplicity over performance. If we'd care about performance, it's probably better to use an approach similar to that of the C reference implementation (as it works entirely on the stack, while the current code does several allocations: 1 in ExpandHRP, between 1 and 3 in CreateChecksum, and 1 inevitable one in Encode).