we already decided this is how attributes should work

not sure why this wasn't already done
start removing usages of 'storage' (#105)
add api for weakening attribute values (#105)

also remove the remaining usages of 'storage' from the test suite
make traits stop using storage (#105)
stop documenting usage of ->storage (#105)
remove last usage of storage (#105)
remove usages of the storage accessor from mop::attribute (#105)