Improve "Inline Function"

[nicoespeon]

Current behavior is very basic. It works for simple cases.

I found myself trying to inline functions sometimes. Most of the time, the refactoring can't be executed. It would be nice to give this one more love, handle more involved scenarios.

There's likely an opportunity to revisit the implementation and simplify it.

[jplace]

+1 to this! The ability to inline a function body into everywhere it's called would be massively helpful. Thank you for your hard work on this tool!