It's a weird function: it lets you modify the token stream in the middle
of iteration. There is only one call site, and it is only used for the
rare `ProceduralMasquerade` legacy case.

It's unused.

By inlining it into the only non-test call site. The one test call site
is changed to use `TokenStreamBuilder`.

Both functions do some modifying of streams using `make_mut`:
- `push` sometimes glues the first token of the next stream to the last
  token of the first stream.
- `build` appends tokens to the first stream.

By doing all of this in the one place, things are simpler. The first
stream can be modified in both ways (if necessary) in the one place, and
any next stream with the first token removed doesn't need to be stored.

Because that's the standard way of doing it.