Extend the lifetime in generated code.

[abizjak]

Purpose

Extend the lifetime in the generated code so that if the return value of the call has references that refer to host the generated code still compiles.

In particular this comes up in advanced cases where we have an entrypoint of a contract of the form

fn receive(...) -> Result<impl Serial, SomeError>

where the return value impl Serial would typically refer to either the host (via state if it is retrieving the state) or to the parameter.

This was discovered while prototyping Concordium/concordium-rust-smart-contracts#386

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.