The strings will get deduplicated when actually turned into
PrimitiveString objects at runtime anyway, and keeping the string
tables deduplicated was actually wasting a lot of time.
4.4% speed-up on Kraken/stanford-crypto-ccm.js :^)
DeprecatedFlyString relies heavily on DeprecatedString's StringImpl, so
let's rename it to A) match the name of DeprecatedString, B) write a new
FlyString class that is tied to String.
This is a specialized string table for storing identifiers only.
Identifiers are always FlyStrings, which makes many common operations
faster by allowing O(1) comparison.
