Cleanup in BoundMultipartStream (#35)
Cleanup in BoundMultipartStream (#35)

Start to make the code a little easier to understand, and fix some
style.

- Initialize the `_controller` field in place and make it final. Set the
  callbacks in the constructor rather than instantiate in the
  constructor. This will prevent it from needing to be `late`.
- Assign `boundaryPrefix`, `contentStartIndex`, and `byte` variables
  where they are declared rather than separate the declaration and
  initialization.

Style fixes:
- Use doc comments.
- Remove unnecessary parens in while loop condition.
- Remove unnecessary library directive.
dart-lang/mime