src/: Yet more naming and export twiddles.

src/: Yet more naming and export twiddles.

  * Export `class-item', which names a pluggable parser invoked in
    parsing class definitions.

  * Export the accessors for `code-fragment-item' objects.

  * Rename the `message-*' generic methods to be `sod-message-*', for
    consistency with the existing accessors.  Specific targets are
    `message-combination', on aggregating messages, and
    `message-effective-method-class', part of the layout protocol, which
    is just inconsistent with `sod-message-method-class' in the
    metaobject construction protocol.

  * Also export `sod-message-kernel-function', which is used in
    aggregating messages.

  * Astonishingly, I'd not exported any of the `vtmsgs' symbols.