This fixes instance_exec and similar methods. It also fixes
Enumerator::Yielder#yield, rb_yield_block, and a couple of cases
with Proc#{<<,>>}.

This support requires the addition of rb_yield_values_kw, similar to
rb_yield_values2, for passing the keyword flag.

Unlike earlier attempts at this, this does not modify the rb_block_call_func
type or add a separate function type.  The functions of type
rb_block_call_func are called by Ruby with a separate VM frame, and we can
get the keyword flag information from the VM frame flags, so it doesn't need
to be passed as a function argument.

These changes require the following VM functions accept a keyword flag:

* vm_yield_with_cref
* vm_yield
* vm_yield_with_block