FIX: Make JsBoundMethod a subclass of JsProxy

[hoodmane]

Fixes #788 (tested in test_nested_attribute_access) and fixes #1123 (tested in test_window_isnt_super_weird_anymore). This also resolves a problem that blocks any fix for #768.

[rth]

This is a very useful, thanks!

If this the fix for #788 why is test_nested_attribute_access passing in #1126 ? Generally there is some diff overlap with that PR: which one should be merged first?

[hoodmane]

Ugh yes, there's several closely bound together bugs here. I merged this one into #1126 in order to make the #1126 tests pass (otherwise a bunch of tests fail). Commit 709118c on that branch has everything needed to fix the bug, the stuff on #1126 after that is I merged this, merged master, and messed around with tests.

So merge this one first and then I can merge master on that one and then merge that one? Also I have some tests that will only pass once this, #1126, and #1033 all are merged. I guess I can add them to the last one to merge (if I can remember where I put them...).

I think between this one and #1033 what should happen is that the Vectorcall implementation in #1033 should be moved to JsBoundMethod, the implementation of JsProxy_Vectorcall should be removed so that JsProxy is no longer callable (I don't think it's possible to get a javascript function anymore that isn't a JsBoundMethod) and then rename JsBoundMethod to JsProxy_Callable. (Also the .new method should be moved to a subclass of JsProxy_Callable which I'd call JsProxy_Constructible, but that could happen separately after everything else).

So I think the right order is this, then #1126, then I can update #1033 as described and merge that third.

[hoodmane]

Oh yeah #1098 is also blocked on this -- after #1124 and possibly #1126 are merged, the test suite for #1098 should start passing. I guess I'll try merging this on #1098 now and see what happens...

[hoodmane]

Also finishing #972 is blocked on this.

[rth]

Thanks! Generally LGTM, but I'm not the most relevant reviewer for C code.

Please add a changelog entry.