[PRISM] Add function to free scope node

pm_scope_node_destroy frees the scope node after we're done using it to
make sure that the index_lookup_table is not leaked.

For example:

    10.times do
      100_000.times do
        RubyVM::InstructionSequence.compile_prism("begin; 1; rescue; 2; end")
      end

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

Before:

    33056
    50304
    67776
    84544
    101520
    118448
    135712
    152352
    169136
    186656

After:

    15264
    15296
    15408
    17040
    17152
    17152
    18320
    18352
    18400
    18608

0 files changed, 0 insertions, 0 deletions