Get rid of `sorted` in bindings rewrites

In #515, @majosm asks: `# FIXME: Are these sorts still necessary?` in relation to code like this:
```py
... for name, subexpr in sorted(expr.bindings.items())}
```
I believe that we can now do without those, so we should try to get rid of them. (post #515)

Search the bindings rewrites for occurrences of sorted(expr.bindings.items()) and inspect the related rewrite tests. Determine whether removing the sorts preserves the expected results, then run the affected tests; done means the unnecessary sorts are gone and the tests still pass.

Written by the indexing model from the issue text.