This PR implements support for NTLM hashes as announced by Troy Hunt:
https://s.pebcak.de/@troyhunt@infosec.exchange/109833758367903768
For this we needed to be able to calculate MD4 hashes, as NTLM basically is calculated like this: `MD4(UTF-16LE(pw))`. For this we ported the official golang.org/x/crypto/md4 package, so we can still claim that "only depends on Go stdlib"
A new Client option has been introduced: `WithPwnedNTLMHash`. If the client is initalized with this option, all generic methods (`ListHashesPassword` and `CheckPassword`) will operate on NTLM hashes.
Additionally, there are now equivalent methods for checking passwords and listing hashes for NTLM: `CheckNTLM` and `ListHashesNTLM`
