Add WriteHDKeypath function and move *HDKeypath to util/bip32.{h,cpp}

Creates new files util/bip32.h and util/bip32.cpp for containing
BIP 32 stuff.
Moves FormatKeyPath from descriptor.cpp to util/bip32.
Adds a wrapper around it to prepent the 'm' for when just the
BIP 32 style keypath is needed.

src/util/strencodings.h

@@ -197,9 +197,6 @@ bool ConvertBits(const O& outfn, I it, I end) {
     return true;
 }
 
-/** Parse an HD keypaths like "m/7/0'/2000". */
-NODISCARD bool ParseHDKeypath(const std::string& keypath_str, std::vector<uint32_t>& keypath);
-
 /**
  * Converts the given character to its lowercase equivalent.
  * This function is locale independent. It only converts uppercase