[Bug #20649] Allow `nil` as 2nd argument of `assign_error`

	Fallback to the last token element in that case, for the backward
	compatibilities.

Our current implementation of rb_postponed_job_register suffers from
some safety issues that can lead to interpreter crashes (see bug #1991).
Essentially, the issue is that jobs can be called with the wrong
arguments.

We made two attempts to fix this whilst keeping the promised semantics,
but:
  * The first one involved masking/unmasking when flushing jobs, which
    was believed to be too expensive
  * The second one involved a lock-free, multi-producer, single-consumer
    ringbuffer, which was too complex

The critical insight behind this third solution is that essentially the
only user of these APIs are a) internal, or b) profiling gems.

For a), none of the usages actually require variable data; they will
work just fine with the preregistration interface.

For b), generally profiling gems only call a single callback with a
single piece of data (which is actually usually just zero) for the life
of the program. The ringbuffer is complex because it needs to support
multi-word inserts of job & data (which can't be atomic); but nobody
actually even needs that functionality, really.

So, this comit:
  * Introduces a pre-registration API for jobs, with a GVL-requiring
    rb_postponed_job_prereigster, which returns a handle which can be
    used with an async-signal-safe rb_postponed_job_trigger.
  * Deprecates rb_postponed_job_register (and re-implements it on top of
    the preregister function for compatability)
  * Moves all the internal usages of postponed job register
    pre-registration

All kind of AST nodes use same struct RNode, which has u1, u2, u3 union members
for holding different kind of data.
This has two problems.

1. Low flexibility of data structure

Some nodes, for example NODE_TRUE, don’t use u1, u2, u3. On the other hand,
NODE_OP_ASGN2 needs more than three union members. However they use same
structure definition, need to allocate three union members for NODE_TRUE and
need to separate NODE_OP_ASGN2 into another node.
This change removes the restriction so make it possible to
change data structure by each node type.

2. No compile time check for union member access

It’s developer’s responsibility for using correct member for each node type when it’s union.
This change clarifies which node has which type of fields and enables compile time check.

This commit also changes node_buffer_elem_struct buf management to handle
different size data with alignment.

Keep the other types, which not having setter macros for ripper.