... if not, these functions are not relevant
to the issue at hand. If the search by truename_is_ significant, we
could cache that as well.
Just to make sure that we are on the same page: the cache I am proposing
should be complete - if a buffer is missing from the cache, we should be
sure that there is no matching buffer.
Since we will keep buffer-list (we must), even with this cache
available, we can always leave the current code that scans the buffer
list if the name is not in the cache. This way, we don't need to
worry to have all the buffers in the cache, only those which are
looked for frequently and need the efficiency.