loci: replace validator with a stub

The negative unit tests are commented out since the stub validator doesn't do
anything.
loci: generate validator class hierachies

Includes object length checks. Does not validate lists yet.
loci: validate fixed-offset non-field-length lists

This is enough to pass all the existing unit tests.
loci: read and validate field-length members
loci: validate lists that use field-length members
loci: add TODOs to validator
loci: remove unused validator import
Merge into master from pull request #245:
Rewrite LOCI validator (https://github.com/floodlight/loxigen/pull/245)