ConvertBits() - convert from one power-of-2 number base to another.

Function extracted from upstream:
  PR bitcoin/bitcoin#11167
  Commit c091b99379b97cb314c9fa123beabdbc324cf7a4

src/utilstrencodings.h

@@ -133,4 +133,28 @@ bool TimingResistantEqual(const T& a, const T& b)
  */
 bool ParseFixedPoint(const std::string &val, int decimals, int64_t *amount_out);
 
+/** Convert from one power-of-2 number base to another. */
+template<int frombits, int tobits, bool pad, typename O, typename I>
+bool ConvertBits(O& out, I it, I end) {
+    size_t acc = 0;
+    size_t bits = 0;
+    constexpr size_t maxv = (1 << tobits) - 1;
+    constexpr size_t max_acc = (1 << (frombits + tobits - 1)) - 1;
+    while (it != end) {
+        acc = ((acc << frombits) | *it) & max_acc;
+        bits += frombits;
+        while (bits >= tobits) {
+            bits -= tobits;
+            out.push_back((acc >> bits) & maxv);
+        }
+        ++it;
+    }
+    if (pad) {
+        if (bits) out.push_back((acc << (tobits - bits)) & maxv);
+    } else if (bits >= frombits || ((acc << (tobits - bits)) & maxv)) {
+        return false;
+    }
+    return true;
+}
+
 #endif // BITCOIN_UTILSTRENCODINGS_H