Mysql.Prepared

`sig
type stmt
type stmt_result
val create : Mysql.dbd -> string -> Mysql.Prepared.stmt
val execute :
Mysql.Prepared.stmt -> string array -> Mysql.Prepared.stmt_result
val execute_null :
Mysql.Prepared.stmt -> string option array -> Mysql.Prepared.stmt_result
val affected : Mysql.Prepared.stmt -> int64
val insert_id : Mysql.Prepared.stmt -> int64
val real_status : Mysql.Prepared.stmt -> int
val fetch : Mysql.Prepared.stmt_result -> string option array option
val result_metadata : Mysql.Prepared.stmt -> Mysql.result
val close : Mysql.Prepared.stmt -> unit
end`