partial bitcoin#19660: refactor: Make HexStr take a span

Modified some code not backported yet

0a8aa62 refactor: Make HexStr take a span (Wladimir J. van der Laan)

Pull request description:

  Make `HexSt`r take a span of bytes, instead of an awkward pair of templated iterators. This simplifies most of the uses.

ACKs for top commit:
  elichai:
    Code review ACK 0a8aa62
  hebasto:
    re-ACK 0a8aa62
  jonatack:
    re-ACK 0a8aa62

Tree-SHA512: 6e178ece5cbac62119c857a10299b1e85422938084c3f03063e17119a5129e0c28016e05a6fabaa4c271a7e0a37c7cd89fa47c435ee19b38a5acfe80d00de992

src/rpc/rawtransaction.cpp

@@ -1583,7 +1583,7 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
 
     if (complete && extract) {
         ssTx << mtx;
-        result_str = HexStr(ssTx.str());
+        result_str = HexStr(ssTx);
         result.pushKV("hex", result_str);
     } else {
         ssTx << psbtx;

src/script/descriptor.cpp

@@ -64,7 +64,7 @@ class OriginPubkeyProvider final : public PubkeyProvider
 
     std::string OriginString() const
     {
-        return HexStr(std::begin(m_origin.fingerprint), std::end(m_origin.fingerprint)) + FormatKeyPath(m_origin.path);
+        return HexStr(m_origin.fingerprint) + FormatKeyPath(m_origin.path);
     }
 
 public:

src/util/strencodings.cpp

@@ -629,3 +629,16 @@ std::string Capitalize(std::string str)
     str[0] = ToUpper(str.front());
     return str;
 }
+
+std::string HexStr(const Span<const uint8_t> s)
+{
+    std::string rv;
+    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]);
+    }
+    return rv;
+}