The sense of the signed bit in the type stream was reversed

Use FLEXI-STREAMS:WITH-INPUT-FROM-SEQUENCE and
FLEXI-STREAMS:WITH-OUTPUT-TO-SEQUENCE where appropriate.

Convert the old function into the default implementation.
Add an abstraction-breaking implementation for FLEXI-STREAMS::VECTOR-INPUT-STREAM.

Add default implementation.

Use WITH-ACCESSORS when breaking the flexi-streams abstraction for the
FLEXI-STREAMS::VECTOR-INPUT-STREAM specialization.

It's not useful without a MYSQL-STATEMENT-SEND-LONG-DATA
implementation, which we don't bother exposing yet.