merge revision(s) 82b57d7b: [Backport #20162]

	Fix memory leak when duplicating too complex object

	[Bug #20162]

	Creating a ST table then calling st_replace leaks memory because the
	st_replace overwrites the ST table without freeing any of the existing
	memory. This commit changes it to use st_copy instead.

	For example:

	    RubyVM::Shape.exhaust_shapes

	    o = Object.new
	    o.instance_variable_set(:@a, 0)

	    10.times do
	      100_000.times { o.dup }

	      puts `ps -o rss= -p #{$$}`
	    end

	Before:

	    23264
	    33600
	    42672
	    52160
	    61600
	    71728
	    81056
	    90528
	    100560
	    109840

	After:

	    14752
	    14816
	    15584
	    15584
	    15664
	    15664
	    15664
	    15664
	    15664
	    15664
	---
	 object.c                 |  3 +--
	 test/ruby/test_shapes.rb | 13 +++++++++++++
	 2 files changed, 14 insertions(+), 2 deletions(-)