Support TS 3.8 with full type safety

[nicojs]

Due to microsoft/TypeScript#37400 it is allowed to assign a parent injector to a child injector.

const fooInjector: Injector<{foo: string}> = rootInjector;
// ERROR in TS <3.8
// OK According to TS >= 3.8

fooInjector.resolve('foo'); // Breaks at runtime!!

I'll close this issue once it's fixed at TS side.