Merge bitcoin/bitcoin#21173: util: faster HexStr => 13% faster blockToJSON

74bf850ac4 faster HexStr => 13% faster blockToJSON (Martin Ankerl)

Pull request description:

  `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.

ACKs for top commit:
  laanwj:
    Code review ACK 74bf850ac4
  theStack:
    re-ACK 74bf850ac4

Tree-SHA512: fc99105123edc11f4e40ed77aea80cf7f32e49c53369aa364b38395dcb48575e15040b0489ed30d0fe857c032a04e225c33e9d95cdfa109a3cb5a6ec9a972415

src/util/strencodings.cpp

@@ -593,13 +593,14 @@ std::string Capitalize(std::string str)
 
 std::string HexStr(const Span<const uint8_t> s)
 {
-    std::string rv;
+    std::string rv(s.size() * 2, '\0');
     static constexpr char hexmap[16] = { '0', '1', '2', '3', '4', '5', '6', '7',
                                          '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
-    rv.reserve(s.size() * 2);
-    for (uint8_t v: s) {
-        rv.push_back(hexmap[v >> 4]);
-        rv.push_back(hexmap[v & 15]);
+    auto it = rv.begin();
+    for (uint8_t v : s) {
+        *it++ = hexmap[v >> 4];
+        *it++ = hexmap[v & 15];
     }
+    assert(it == rv.end());
     return rv;
 }