- make closures know where they should be executed (eg, on a workqueue,
  or a combiner, or on an exec_ctx)
- this allows removal of a large number of trampoline functions that
  were appearing whenever we used combiners, and should allow for a much
  easier interface to combiner locks

- Initializes grpc correctly in the JWT utils.
- Make the email mapping work with the new service accounts produced by
  Google IAM.
- Adding check for email issuers where the issuer has to be the subject as well.
- Implementing portable version of memrchr.

Documentation for the new tests can be found https://github.com/grpc/grpc/blob/master/doc/negative-http2-interop-test-descriptions.md

Uses dynamic loading to paper-over the negative effects of losing
namespace packages in the previous commit.

Uses dynamic loading to paper-over the negative effects of losing
namespace packages in the previous commit.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.