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.

src/util/strencodings.cpp

@@ -579,13 +579,14 @@ 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;
 }