'specialization of template in different namespace'

I haven't seen this error before and I don't know what compiler is causing it, but here's a guess at a fix.

See also https://github.com/python/cpython/issues/91768

Fixes #302

Also stops building the mac wheels we ship with Ofast and debugging assertions. That's not meant for production use.

Based on, and fixes #335

greenlet: Drop using 'register' storage class keyword

The assertions were actually incorrect if the same tracer object was set twice.

Removing the assertions is ok here because the reference handling is pretty thoroughly debugged and tested now.

This has been dropped in c++17 and newer

Signed-off-by: Khem Raj <raj.khem@gmail.com>