The following script leaks memory:

    10.times do
      100_000.times do
        "\ufffd".encode(Encoding::US_ASCII, fallback: proc { Object.new })
      rescue
      end

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

Before:

    450244
    887748
    1325124
    1762756
    2200260
    2637508
    3075012
    3512516
    3950020
    4387524

After:

    12236
    12364
    12748
    13004
    13388
    13516
    13772
    13772
    13772
    13772

The following script leaks memory:

    10.times do
      100_000.times do
        "\ufffd".encode(Encoding::US_ASCII, fallback: proc { "\uffee" })
      rescue
      end

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

Before:

    451196
    889596
    1328252
    1766524
    2204668
    2643068
    3081724
    3520124
    3958524
    4396796

After:

    12800
    13056
    13184
    13312
    13312
    13312
    13312
    13312
    13312
    13312

When the fallback function in transcode_loop raises, it will leak the memory
in rb_econv_t. The following script reproduces the leak:

    10.times do
      100_000.times do
        "\ufffd".encode(Encoding::US_ASCII, fallback: proc { raise })
      rescue
      end

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

Before:

    451196
    889980
    1328508
    1767676
    2206460
    2645372
    3083900
    3522428
    3960956
    4399484

After:

    12508
    12636
    12892
    12892
    13148
    13404
    13532
    13788
    13916
    13916

Not all ractor-related encoding issues were fixed by 1afc07e815051e2f73493f055f2130cb642ba12a.
I found more by running my test-all branch with 3 ractors for each test.

Make sure VM lock is not held when calling `load_transcoder_entry`, as
that causes deadlock inside ractors. `String#encode` now works inside
ractors, among others.

Atomic load the rb_encoding_list

Without this, wbcheck would sometimes hit a missing write barrier.

Co-authored-by: John Hawthorn <john.hawthorn@shopify.com>

Hold VM lock when iterating over global_enc_table.names

This st_table can be inserted into at runtime when autoloading
encodings.

minor optimization when calling Encoding.list

Ex: `str.encode` and `str.encode!` work across ractors now.

The global table `transcoder_table` needs a lock around each st_lookup/st_insert, and it's a two-level
table so the second level also needs to be locked around insertion/deletion. In addition to this, the
transcoder entries (values in the second-level hash table) need to be locked around retrieving them and
loading them as they are loaded lazily.

The transcoding objects (`Encoding::Converter`) can't be made shareable, so their operations don't need to be locked.

Delete duplicated `Document-const:` directives

Those directives are attached to `rb_define_const` and `rb_file_const` where we don’t need them because the functions are supported by RDoc.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>