libsyntax: Remove the `use foo = bar` syntax from the language in favor
of `use bar as foo`.

Change all uses of `use foo = bar` to `use bar as foo`.

Implements RFC #47.

Closes #16461.

[breaking-change]
auto merge of #16575 : pcwalton/rust/import-foo-as-bar, r=aturon

of `use bar as foo`.

Change all uses of `use foo = bar` to `use bar as foo`.

Implements RFC #47.

Closes #16461.

[breaking-change]

r? @aturon