Add db module for easy extensiblity

[mnpw]

db::Type -> Enum defining which all database variants are supported
db::Insertor -> Trait specifying insert related methods that will be required by any new db we wish to plug
db::Database -> Enum to hold database objects corresponding to db::Type

Support for a NewDb would look like following

    pub enum Type {
        ...
+       NewDb,
    }
    
    pub enum Database {
        ...
+       NewDb,
    }
    
    impl Database {
        pub fn new(db_type: &Type, options: ClientOptions, table: &str) -> Database {
            match db_type {
                ...
+               Type::NewDb => Self::NewDb(NewDb::new(options, table)),
            }
        }
    }
    
+   struct NewDb {
+       ...
+   }
+   
+   impl NewDb {
+       pub fn new(...) -> NewDb {
+           ...
+       }
+   }
+   
+   impl Inserter for NewDb {
+       ...
+   }