`val reserve : Ppxlib__.Import.string ‑> Ppxlib__.Import.unit`

`reserve "foo"` has two implications:

- one can't then declare an attribute inside this namespace
- attributes within this namespace won't be reported by `check_unused`

This is here to insure that the rewriter cohabits well with other rewriter or tools (e.g. merlin) which might leave attribute on the AST.

N.B. the "merlin" namespace is reserved by default.

`val is_in_reserved_namespaces : Ppxlib__.Import.string ‑> Ppxlib__.Import.bool`