support features like #![feature(async_closure)]

[Dominic Burkart]

Currently, turbolift modifies the AST for the source file of main.rs in several ways, including prepending some use statements. This is problematic for several reasons:

• it's bad hygeine and could cause conflicts.
• prepending breaks support for feature declarations.

[Dominic Burkart]

Feature declarations were added to the example tests to avoid regression.