Use `Symbol` rather than `OpaqueToken`

Given the use case for OpaqueToken it seems like you could do away with that type and use Symbol instead. Symbols are immutable and unique, and the polyfills for Symbol are generally lightweight.

More information on Symbol

[benlesh]

As mentioned in Slack:

Advantages to using Symbol:

• Symbols are completely unique and immutable, no one else’s module can collide with your symbol.
• Symbols can be used directly as keys to hash tables (might improve your DI perf, depending on your impl)
• You’ll be reusing a type that already exists in the language
• You can optionally provide symbols that can be shared globally with Symbol.for.

[benlesh]

@tbosch so this runtime class exists to support build-time typing ergonomics? That doesn’t seem good.

Why not just allow generics on the get method?

const symbol = Symbol.for('foo');
const thing = Injector.get<SomeType>(symbol);

vs

const token = new OpaqueToken<SomeType>('foo');
const thing = Injector.get(token);