Add valid functions

```haskell
-- check invariants
valid :: Text -> Bool
```

For strict text it would check that array is big enough to have offset+length, and that the slice is valid utf8.

For lazy text it would check the above, and chunk invariants.

These functions would be very useful (in writing tests) when one tinkers with internals. (Either in `text` itself, or other packages).

There are `strictInvariant` and `lazyVariant` for lazy text, but they `error`. Not entirely, but almost useless in tests.