Make `InMemorySigner` parameter-fetching utilities return `Option`s

Make `InMemorySigner` parameter-fetching utilities return `Option`s

In the previous commit we fixed a bug where we were spuriously
(indirectly) `unwrap`ing the `channel_parameters` in
`InMemorySigner`. Here we make such bugs much less likely in the
future by having the utilities which do the `unwrap`ing internally
return `Option`s instead.

This makes the `unwrap`s clear at the callsite.