Implement unicode support for lazily encoding strings

[Vizonex]

Something I've come across when using this library is wanting to make things easier for users hashpw(password.encode('utf-8'), ...) can be rather annoying and I hope this PR tries to optimize some of the brokenness with unicode support. I'm sure the example I've given logically illustrates what I am after.

from fastapi import FastAPI
from bcrypt import checkpw_from_string

app = FastAPI()

@app.post("/login")
def login(username:str, password:str):
        # SQL Query performed and is fetching the username and bcrypt hash
        ... = checkpw_from_string(password, _hash)

[alex]

KDFs operate over byte strings, unless the KDF itself defines an encoding for strings, I think offering this API is a mistake, because it misleads users into not understanding what level of abstraction things operate at. The extremely modest inconveience of having to encode your string is worth it to keep thing conceptually correct.