Dynamic import declarations and expressions

[edemaine]

Fixes #727:

• import { getX } from library → const { getX } = await import('library') if not at the top level
• import lib, {getX} from library → const lib = await import('library'), {getX} = lib if not at the top level
• stuff := import { getX, getY } from library → const stuff = (await import('library')).{getX, getY}

This complements existing functionality of stuff := import 'library' → stuff := import('library'). Note that stuff := import library has a different meaning than above (use the variable library instead of the string).

I haven't implemented assert/with yet. I probably should... Import assertions via assert/with should work now.

Also fixes a bug where {x: y} := obj got implicitly returned as {x}, but it should be {y} (!).

[bbrk24]

import lib, {getX} from library → const lib = await import('library'), {getX} = lib if not at the top level

Shouldn't this be

const ref = await import('library'),
  { getX, default: lib } = ref;

?

[STRd6]

Thanks, this looks like a very useful feature that makes dynamic imports much easier to use.