Fix exception during Hash#each and Hash#each_key if keys get deleted during loop

[janbiedermann]

Given the following ruby:

o = { a: 1, b: 2, c: 3 }
h = o.dup
h.each { |k,v| h.delete(k) }
i = o.dup
i.each_key { |k| i.delete(k) }
puts "o: #{o}\nh: #{h}\ni: #{i}"

and executed with matz ruby results in the following expected output:

o: {:a=>1, :b=>2, :c=>3}
h: {}
i: {}

However, using opal master the following exceptions are thrown (need to comment out the .each ain above code to get the second exception):

<internal:corelib/hash.rb>:429:1:in `each': Cannot read properties of undefined (reading '$$is_string') (Exception)
        from <internal:corelib/runtime.js>:1900:5:in `Opal.send2'
        from <internal:corelib/runtime.js>:1888:5:in `each'
        from test.rb:3:2:in `undefined'
        from <internal:corelib/runtime.js>:2760:7:in `<main>'
        from test.rb:1:1:in `null'
        from node:internal/modules/cjs/loader:1103:14:in `Module._compile'
        from node:internal/modules/cjs/loader:1155:10:in `Module._extensions..js'
        from node:internal/modules/cjs/loader:981:32:in `Module.load'
        from node:internal/modules/cjs/loader:822:12:in `Module._load'
        
<internal:corelib/hash.rb>:450:1:in `each_key': Cannot read properties of undefined (reading '$$is_string') (Exception)
        from <internal:corelib/runtime.js>:1900:5:in `Opal.send2'
        from <internal:corelib/runtime.js>:1888:5:in `each_key'
        from test.rb:5:2:in `undefined'
        from <internal:corelib/runtime.js>:2760:7:in `<main>'
        from test.rb:1:1:in `null'
        from node:internal/modules/cjs/loader:1103:14:in `Module._compile'
        from node:internal/modules/cjs/loader:1155:10:in `Module._extensions..js'
        from node:internal/modules/cjs/loader:981:32:in `Module.load'
        from node:internal/modules/cjs/loader:822:12:in `Module._load'

This happens because the original $$keys aray is used for the loop. If that gets modified during the loop, the indexes fail to match during the loop, resulting in null keys and failing.

The patch copies the $$keys using .slice() (the supposedly fastest way to copy arrays) before entering the loop:

o: {"a"=>1, "b"=>2, "c"=>3}
h: {}
i: {}

[hmdne]

Thanks! Let's merge!