The previous changes in #33 improved memory consumption for settled promises somewhat. Similarly, this PR addresses memory consumption for pending promises that are no longer referenced.

Calling timeout() on a promise which has no canceller function does successfully reject the promise. However, due to internal references, it keeps a cyclic reference to the input promise and as such shows some unexpected memory consumption and memory would not immediately be freed as expected. Let's not call this a "memory leak", because memory was eventually freed, but this clearly caused some unexpected and significant memory growth.

I'm marking this PR as WIP because this includes a test for reactphp/promise#124 which is yet to be released as part of react/promise v2.7.0. Once this release is out, I'll update the version reference and this should be ready to be shipped.

Builds on top of #33