Merge bitcoin/bitcoin#28172: refactor: use string_view for passing string literals to Parse{Hash,Hex}

bb91131d54 doc: remove out-of-date external link in src/util/strencodings.h (Jon Atack)
7d494a48dd refactor: use string_view to pass string literals to Parse{Hash,Hex} (Jon Atack)

Pull request description:

  as `string_view` is optimized to be trivially copiable, whereas the current code creates a `std::string` copy at each call.

  These utility methods are called by quite a few RPCs and tests, as well as by each other.

  ```
  $ git grep "ParseHashV\|ParseHashO\|ParseHexV\|ParseHexO" | wc -l
  61
  ```

  Also remove an out-of-date external link.

ACKs for top commit:
  jonatack:
    Rebased per `git range-diff c9273f6 b94581a bb91131` for an include header from the merge of https://github.com/bitcoin/bitcoin/pull/28230. Should be trivial to re-ACK.
  maflcko:
    lgtm ACK bb91131d54
  ns-xvrn:
    ACK bb91131d54
  achow101:
    ACK bb91131d54
  brunoerg:
    crACK bb91131d54

Tree-SHA512: 9734fe022c9e43fd93c23a917770d332dbbd3132c80a234059714c32faa6469391e59349954749fc86c4ef0b18d5fd99bf8f4b7b82d9f799943799c1253272ae

src/util/strencodings.h

@@ -260,7 +260,6 @@ bool TimingResistantEqual(const T& a, const T& b)
 }
 
 /** Parse number as fixed point according to JSON number syntax.
- * See https://json.org/number.gif
  * @returns true on success, false on error.
  * @note The result must be in the range (-10^18,10^18), otherwise an overflow error will trigger.
  */