In Context 0.2, return to splitting IdentifierName, PropertyKey and LiteralString

[David Teller]

We had this implemented in Entropy and it made the file smaller. The machinery is still around, it shouldn't be too hard to use it for testing this in Context 0.2 once we're done porting Context 0.1 to Rust.

Also, experiments with the SpiderMonkey implementation suggest that this would make decoding faster.