Upgrade ffi to 2.0.1 and ffigen to 6.0.1

[aospiridonov]

Upgrade with ffi 2.0.1 and ffigen 6.0.1.

But need upgrade:

• cronet to actual version
• upload new release files (libs: libcronet and libwrapper)
• edit ./lib/src/constants.dart to new tag, cronetVersion, wrapperVersion

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.