The small symbolization

[hmdne]

Ref: #2144

This PR will explore ways to gradually move to new API that sets method keys using JavaScript symbols and make that API available before we do the full migration.

[hmdne]

 Comparison of the Asciidoctor (a real-life Opal application) compile and run:
                  Compile time: 4.536 (+25.22%) -> 4.490 (+24.91%) (change: -1.01%)
                      Run time: 1.224 (+8.87%) -> 1.223 (+36.85%) (change: -0.02%)
                   Bundle size: 4625.64 kB -> 4718.50 kB (change: +2.01%)
          Minified bundle size: 990.33 kB -> 1074.65 kB (change: +8.51%)
            Mangled & minified: 698.46 kB -> 687.68 kB (change: -1.54%)

The performance is almost the same (even though we haven't introduced real Symbols yet). Bundle size increase for minified is expected.