v18.0.0

Breaking changes:

• Conn::{query, first, *_exec, ...} were removed in favor of Queryable trait (see crate docs);
• Pool::{prepare, *_exec, ...} were removed in favor of TextQuery and BinQuery traits (see crate docs);
• helpers for batch statement execution were added in forms of Queryable::batch_exec method and BatchQuery trait (see crate docs);
• Stmt was removed in favor of the new Statement structure, which is a thin wrapper around statement identifier. There are few caveats, please consult the corresponding section in the crate docs;
• OptsBuilder and SslOpts methods now takes self by value;
• QueryResult interface was altered;
• start_transaction now takes new TxOpts structure;

Fixes #189, #198, #199, #203, #204.