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.