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

0 files changed, 0 insertions, 0 deletions