ruby2_keywords warnings: Quote non-UTF8 method names fully

	It used to quote only part of the method name because NUL byte in
	the method terminates the C string:

	```
	(irb)> "abcdef".encode("UTF-16LE").bytes
	=> [97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102, 0]
	```

	```
	expected: /abcdef/
	actual: warning: Skipping set of ruby2_keywords flag for a (method not defined in Ruby)\n".
	```

	Do not respect ruby2_keywords on method/proc with post arguments

	Previously, ruby2_keywords could be used on a method or proc with
	post arguments, but I don't think the behavior is desired:

	```ruby
	def a(*c, **kw) [c, kw] end
	def b(*a, b) a(*a, b) end
	ruby2_keywords(:b)

	b({foo: 1}, bar: 1)
	```

	This changes ruby2_keywords to emit a warning and not set the
	flag on a method/proc with post arguments.

	While here, fix the ruby2_keywords specs for warnings, since they
	weren't testing what they should be testing.  They all warned
	because the method didn't accept a rest argument, not because it
	accepted a keyword or keyword rest argument.

rb_obj_setup: do not copy RUBY_FL_SEEN_OBJ_ID

	[Bug #20250]

	We're seting up a new instance, so it never had an associated
	object_id.

	proc.c: get rid of `CLONESETUP`
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	[Bug #20253]

	All the way down to Ruby 1.9, `Proc`, `Method`, `UnboundMethod`
	and `Binding` always had their own specific clone and dup routine.

	This caused various discrepancies with how other objects behave
	on `dup` and `clone. [Bug #20250], [Bug #20253].

	This commit get rid of `CLONESETUP` and use the the same codepath
	as all other types, so ensure consistency.

	NB: It's still not accepting the `freeze` keyword argument on `clone`.

	Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

Previously, Kernel#lambda returned a non-lambda proc when given a
non-literal block and issued a warning under the `:deprecated` category.
With this change, Kernel#lambda will always return a lambda proc, if it
returns without raising.

Due to interactions with block passing optimizations, we previously had
two separate code paths for detecting whether Kernel#lambda got a
literal block. This change allows us to remove one path, the hack done
with rb_control_frame_t::block_code introduced in 85a337f for supporting
situations where Kernel#lambda returned a non-lambda proc.

[Feature #19777]

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>