Add locks around accesses/modifications to global encodings table

This fixes segfaults and errors of the type "Encoding not found" when
using encoding-related methods and internal encoding c functions across
ractors.

Example of a possible segfault in release mode or assertion error in debug mode:

```ruby
rs = []
100.times do
  rs << Ractor.new do
    "abc".force_encoding(Encoding.list.shuffle.first)
  end
end
while rs.any?
  r, obj = Ractor.select(*rs)
  rs.delete(r)
end
```

0 files changed, 0 insertions, 0 deletions