Merge `TokenStreamBuilder::push` into `TokenStreamBuilder::build`.

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.

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