Support async iteration statement for-await-of introduced in ES2018

[twada]

Generates for-await-of statement using await property of ForOfStatement.

async function f() {
    for await (const x of ait) {
        console.log(x);
    }
}

refs: https://tc39.github.io/proposal-async-iteration/
refs: https://github.com/tc39/proposal-async-iteration#the-async-iteration-statement-for-await-of
refs: estree/estree#185

[twada]

@michaelficarra Would you review this pull-req?

[michaelficarra]

LGTM. I'll wait until estree/estree#185 is merged before committing to this AST structure.

[twada]

@michaelficarra Thank you for merging this. Would you cut a new (maybe a minor) release?

[michaelficarra]

@twada Published 1.10.0: df61463

[twada]

@michaelficarra Thanks!

[sanex3339]

Hi. With compact: true escodegen produced wrong code:

forawait

instead of

for await

[sanex3339]

Fix:
#402