Revisit `filesystem`, `internal`, `external`, and `locale` encodings

[kddnewton]

These encoding aliases are actually dynamic, and can be changed by user code. For example if you have test.rb:

Encoding.default_external = Encoding::Windows_1253
require_relative "test2"
p $foo.encoding

and test2.rb:

# encoding: external
$foo = "\xb5"

then the whole thing will print #<Encoding:Windows-1253>. Currently we alias a lot of these to UTF-8, and don't handle internal at all. This whole thing needs to be revisited to allow consumers to pass in their encodings.

[eregon]

Oh wow, is anyone using these encoding in the magic encoding comment? I have never seen it.

[kddnewton]

I both doubt it and hope not

[kddnewton]

Fixed by ruby/ruby#9490