Don't keep pointer to PyString internal buffer
Don't keep pointer to PyString internal buffer

_ctx->name was holding a pointer to a PyString internal buffer, which is
unsafe. The referenced memory can be free and leave a dangling pointer.

Now _ctx->name hold the PyObject returned from the context_name_callback, or
the current thread class name. This also simplify _currnet_context_name().

sumerc/yappi,sumerc/yappi