This reverts commit e3885c5f.

This reverts commit 2046b4ab.

We no longer use the "punkt" tokenizer, instead using the regex-based
tokenizer from wordfreq 1.0. Standardizing functions no longer have
basic versions in `uri` and smarter versions in `nodes`; everything has
been moved into `nodes`.

I've found that the existing normalize_text code is difficult to use
because it may raise a ValueError when given a text such as ','. It's
preferable to replace these strings with '_' so they can still be
represented in a URI, but still allow `valid_concept_name` to check
whether this would happen.

If we restrict URI pieces to those with balanced brackets, this will
prevent a lot of useless URI indexes -- in fact, only complete assertion
URIs will get indexed, which is fine.

metanl was created so it could support both ConceptNet and our
development at Luminoso. The use cases have diverged significantly,
however, and it no longer makes sense to support them with a single
package.

ConceptNet used only a small part of metanl, so we can instead
merge it in as the "conceptnet5.language" subpackage.