Hoist import and export declarations to the beginning of the enclosing block

[benjamn]

See these failing tests for examples when hoisting import and export declarations is helpful/necessary. Thanks to @sokra for those tests.

This will require a slightly more sophisticated compilation strategy, since we're no longer just replacing import and export declarations in place, but potentially moving them around. I suspect I will end up using https://www.npmjs.com/package/magic-string, which also provides source maps.

One subtle implication of this change is that imported symbols should not be visible outside the block where they are imported, because we won't be hoisting import declarations outside the block, and the variables are likely to be undefined before the block, so they might as well not be accessible at all outside the block. In other words, this change is definitely a good idea: #17

[benjamn]

If we're hoisting export declarations, we might as well merge multiple exports into one

module.export({
  a: () => a,
  b: () => b,
  ...
});

instead of

module.export("a", () => a);
module.export("b", () => b);

[benjamn]

Fixed by c884634.