faster HexStr => 13% faster blockToJSON

`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.

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