Parameterize the lifetime logic over the user mask.

See merge request iris/lambda-rust!24

By default this parameter is filed by the namespace lft_userN. The old method, hard-wiring this parameter to lft_userN, had the defect of requiring to place libraries' namespacec in it *in advance*. The weak branch has always used such a parameter.

This reverts commit a9f50489.

This reverts commit f4a6ea09.

Improve notations for functions

See merge request iris/lambda-rust!23

We use fnrec: for recursive functions and fn: for non-recursve functions. This is both shorter and closer to Rust.