refactor: Make HexStr take a span

Make HexStr take a span of bytes, instead of an awkward pair of
templated iterators.

src/util/strencodings.cpp

@@ -569,3 +569,16 @@ 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;
+}