In tool.c/tool.h/tool_tests.c
  hd_privkey_wif(hd_privkey, out_privkey_wif, inout_size)
  Convert a BIP32 extended private key to WIF format.

In bip32.c/bip32.h/bip32_tests.c
  dogecoin_hdnode_serialize_privkey_wif(node, chain, outstr, strsize)
  Serialize private key from HD node as WIF private key string.

In chainparams.c/chainparams.h
  chain_from_bip32_prefix(bip32extkey)
  Determine chainparams from BIP32 extended private or public key.

It makes more sense as an address function, and should be available
to language wrappers the same as other address functions.

Added documentation in address.md

Removed dogecoin_hdnode_serialize_privkey_wif.