This reverts commit 48d833a9, reversing
changes made to 66e9d8e8.

As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...

This also fixes the x25519_NEON symbol error when importing the gRPC
Python modules on ARM.

Change-Id: Id98cf6b0f9a3a8f5b88204bd0a6ad2346182ba3d

Added the ability to create channels from client_channel_factory. This
will be used by the grpclb code to create the channels to communicate
with the LB servers.

This extends the existing http parser to support requests as well as responses.

httpcli continues to exist and work as it has previously, though in the new
directory src/core/http (to reflect the fact the directory now contains code
relevant to parsing requests, which httpcli would not generally involve itself
in).