[ruby-core:100148] [Ruby master Feature#17188] Freeze Encoding objects for Ractor

Issue #17188 has been reported by Eregon (Benoit Daloze).

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Feature #17188: Freeze Encoding objects for Ractor
https://bugs.ruby-lang.org/issues/17188

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Target version: 3.0
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Currently Encoding objects are not frozen:
```
$ ruby -ve 'p Encoding::US_ASCII.frozen?'
ruby 3.0.0dev (2020-09-25T08:28:42Z master 81dc37b1b4) [x86_64-linux]
false
```

That means they cannot be accessed in a Ractor:
```ruby
irb(main):001:0> Encoding::US_ASCII
=> #<Encoding:US-ASCII>

irb(main):002:0> Ractor.new { Encoding::US_ASCII }
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
=> #<Ractor:#2 (irb):2 running>
#<Thread:0x00005567521d81e8 run> terminated with exception (report_on_exception is true):
(irb):2:in `block in irb_binding': can not access non-sharable objects in constant Encoding::US_ASCII by non-main Ractor. (NameError)
```

And `Ractor.new { p "".encoding }` is likely violating the Ractor guarantees.

I think we can make all Encoding instances frozen.

Making them frozen is also useful for code sharing, which TruffleRuby aims to support via the GraalVM Engine/Context API.

cc @ko1



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