don't rename default exports unnecessarily

[Rich-Harris]

This scratches an itch I've had for quite some time. Due to the way esperanto avoids variable name conflicts, something like this...

var foo = 42;
export default foo;

...gets transpiled to this inside a bundle:

var foo = 42;
var _foo = foo;

A large enough codebase will be littered with this sort of thing. Or, if foo.js is inside a directory, say utils, it will come out as var utils__foo = foo, or something like that.

Clearly it's unnecessary - we should just use foo throughout. The one exception is where foo is updated or assigned to after it has been exported, since IIRC default bindings aren't live, like regular bindings (can't find the reference now, spotty wifi - correct me if I'm wrong!).

In other words, this...

var foo = 42;
export default foo;
foo++;

...should become this:

var foo = 42;
var _foo = foo;
foo++;

That edge case is dealt with, at the cost of some pretty funky code, which I might try and clean up a little before merging this in.

I've tried this out on a few examples and the difference is huge - the code looks handwritten rather than generated, and is accordingly much easier to read in its bundled state.