fix `compilation.stream.map` test by switching usage of `StackPtr.u`/`.b`
fix `compilation.stream.map` test by switching usage of `StackPtr.u`/`.b`

the code for `StackPtr.u` no longer adheres to the Scaladoc,
but maybe it's easier to change the scaladoc than change the
implementation and all the call sites

unisonweb/platform,unisonweb/platform,unisonweb/platform