ANN&GRAM: Add `async/await` and `try` support

[mchernyavsky]

Closes #2400. Relates to #2622.

[surdus]

Tried this changes. Found that it shows errors for await macro await!(...) and for combination unsafe and async in function declaration unsafe async fn foo() {

[gilescope]

🎅 This is on my xmas list :-)
Currently every time autoformatting kicks in it trashes the code's formatting as it doesn't get the depth right. We don't need too much singing and dancing, but if it could stop mauling the code when it autoformats within an async block that would be stella.

[gfreezy]

auto complete & goto definition don't work for async fn

[mchernyavsky]

@gfreezy Can you provide some example code? Auto-completion & resolve work for me:

• Edition 2015:

• Edition 2018:

[imkow]

Is this in nightly release?

[Undin]

@imkow no, because this PR is not merged yet

[imkow]

@Undin when will this be merged?

[mchernyavsky]

@Undin when will this be merged?

@vlad20012 ping

[mchernyavsky]

id is not identified as a function.

@gfreezy What do you mean? On attached screenshot everything seems ok.

[vlad20012]

bors r+

[bors]

Build succeeded

• continuous-integration/appveyor/branch
• continuous-integration/travis-ci/push

[Undin]

Note, this PR adds only syntax support
Type inference for try expression and all async related things is not implemented yet