faster HexStr => 13% faster blockToJSON

Summary:
std::string's push_back is rather slow because it needs to check & update the string size. For
HexStr the output string size is already easily know, so we can initially create the string with
the correct size and then just assign the data.

HexStr is heavily usd in blockToJSON, so this change is a noticeable benefit. Benchmark on an i7-8700 @3.2GHz:

• 71,315,461.00 ns/op master
• 62,842,490.00 ns/op this commit

So this little change makes blockToJSON about ~13% faster.

This is a backport of core#21173

Test Plan: ninja all check-all bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14396