Make issues for missing features

There are a number of features from folly, and possibly from the spec, that we did not initially port. We should create an issue for each one with more details about the feature, why it's useful, and any pointers for how to port it.

Compare the current port with the corresponding features in folly and the relevant specification. For each missing feature, record why it is useful and any pointers for porting it, then create a separate issue with those details. There are no files or tests named in this issue.

Written by the indexing model from the issue text.