Make into_pam_conv public

[TheDcoder]

Hi,

First of all, thanks for writing this crate, it was a big help in making progress with PAM authentication in my project!

However there's one flaw that I can't get over without forking this crate, The handy into_pam_conv function which turns a native Conversation implementation into a conversation which can be passed on to start is currently not publicly exported.

This extremely limits the scope of what this crate can be used for since Client does not support setting advanced functionality such as set_item, acct_mgmt or chauthtok.