There were some subtle behaviors with the stream being implemented incorrectly & not working as expected with async iteration.  I've modified the code based on #2050 and comments in #2035 to have better test coverage of async iterables and update the internals significantly to more closely match the readable stream interface.

Note: this is a __breaking__ (semver major) change to this package as the close event behavior is changed slightly, and `highWaterMark` is no longer supported.  It shouldn't impact most usage, but breaking regardless.