Include buffer logic to avoid dependency on reactphp/promise-stream

[clue]

This changeset adds buffering logic to avoid the otherwise unneeded dependency on reactphp/promise-stream. There are plans to deprecate that package as discussed in https://github.com/orgs/reactphp/discussions/475 and in either case I'd like to remove additional dependencies where possible.

In particular, reactphp/http appears to be the only ReactPHP package that depends on reactphp/promise-stream at the moment. This means that most consumers of this package would no longer install reactphp/promise-stream at all. Perhaps most notably, this means that IDE autocompletion for all() would now only suggest the correct React\Promise\all() function instead of the rarely useful React\Promise\Stream\all() function.

This changeset does not otherwise affect our public API, so this should be safe to apply. The test suite confirms this has 100% code coverage and does not otherwise affect our APIs. This improves error reporting slightly, but I've tried to keep changes to a minimum otherwise.

Note that this changeset does not preclude the discussion in https://github.com/orgs/reactphp/discussions/475, so whether or not or when the package will be deprecated is still up for debate. If you want to explicitly install this dependency, you can still install it like this:

composer require react/promise-stream

Builds on top of #480, #460 and others
Refs #468