Preallocate result in TryParseHex to avoid resizing

Running `make && ./src/bench/bench_bitcoin -filter=HexParse` a few times results in:
```
|           ns/base16 |            base16/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                0.68 |    1,465,555,976.27 |    0.8% |      0.01 | `HexParse`
|                0.68 |    1,472,962,920.18 |    0.3% |      0.01 | `HexParse`
|                0.68 |    1,476,159,423.00 |    0.3% |      0.01 | `HexParse`
```

Author: Lőrinc

src/util/strencodings.cpp

@@ -81,6 +81,8 @@ template <typename Byte>
 std::optional<std::vector<Byte>> TryParseHex(std::string_view str)
 {
     std::vector<Byte> vch;
+    vch.reserve(str.size() / 2); // two hex characters form a single byte
+
     auto it = str.begin();
     while (it != str.end()) {
         if (IsSpace(*it)) {