EXPERIMENT: Make `async fn`, `.await`, `async move? {` legal on 2015

[Centril]

The parser currently will not parse async fn, async move? $block, and <expr>.await on Rust 2015.

Check whether:

• doing so would simplify the parser & the grammar.
• it is feasible with a crater run for <expr>.await and async move? $block

Drawbacks include:

• folks having fewer incentives to use Rust 2018.