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