Merge bitcoin#29606: refactor: Reserve memory for ToLower/ToUpper con…

…versions

6f2f4a4 Reserve memory for ToLower/ToUpper conversions (Lőrinc)

Pull request description:

  Similarly to bitcoin#29458, we're preallocating the result string based on the input string's length.
  The methods were already [covered by tests](https://github.com/bitcoin/bitcoin/blob/master/src/test/util_tests.cpp#L1250-L1276).

ACKs for top commit:
  tdb3:
    ACK for 6f2f4a4
  maflcko:
    lgtm ACK 6f2f4a4
  achow101:
    ACK 6f2f4a4
  Empact:
    Code Review ACK bitcoin@6f2f4a4
  stickies-v:
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Tree-SHA512: e3ba7af77decdc73272d804c94fef0b11028a85f3c0ea1ed6386672611b1c35fce151f02e64f5bb5acb5ba506aaa54577719b07925b9cc745143cf5c7e5eb262

src/util/strencodings.cpp

@@ -446,13 +446,15 @@ bool ParseFixedPoint(std::string_view val, int decimals, int64_t *amount_out)
 std::string ToLower(std::string_view str)
 {
     std::string r;
+    r.reserve(str.size());
     for (auto ch : str) r += ToLower(ch);
     return r;
 }
 
 std::string ToUpper(std::string_view str)
 {
     std::string r;
+    r.reserve(str.size());
     for (auto ch : str) r += ToUpper(ch);
     return r;
 }