[PRISM] Fix memory leak when compiling file

There is a memory leak when passing a file to
RubyVM::InstructionSequence.compile_prism because it does not free the
mapped file.

For example:

    require "tempfile"

    Tempfile.create(%w"test_iseq .rb") do |f|
      f.puts "name = 'Prism'; puts 'hello'"
      f.close

      10.times do
        1_000.times do
          RubyVM::InstructionSequence.compile_prism(f)
        end

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

Before:

    27968
    44848
    61408
    77872
    94144
    110432
    126640
    142816
    159200
    175584

After:

    11504
    12144
    12592
    13072
    13488
    13664
    14064
    14368
    14704
    15168

0 files changed, 0 insertions, 0 deletions