Move the `gen_salt` function to the Base module

[David Whitlock]

The main Bcrypt module should only contain the functions that the average developer would use, and all of the lower-level functions should be in the Bcrypt.Base module. With this in mind, the gen_salt function should really be in the Bcrypt.Base module, as most users will not use it directly.

I propose the following:

• move the gen_salt function to the Bcrypt.Base module
• deprecate the gen_salt function in the Bcrypt module (and remove it in the next major version)

[David Whitlock]

Updated in 0c865e5