expose Dictionary as part of an API

[alex-okrushko]

When code exports a symbol whose type is inferred (or contains an inferred type), the type must be imported so that a correct name can be generated for it in the resulting .d.ts file.

For example, that means when we create any selector we have to import MemoizedSelector to the file, and, also not to have the unused import linter error, at least one of the selectors has to be fully typed.

Since MemoizedSelector is exported through public_api.ts, solving this problem is pretty straightforward, however one of the teams has started using entities now and has to import Dictionary for the exactly same reason, but Dictionary is not exported.

e.g. this is the code that they need to use:

export const fooEntities: MemoizedSelector<StateWithFoo, Dictionary<Foo>> =
    createSelector(fooFeature, adapter.getSelectors().selectEntities);

for more on exported variables with inferred type:
microsoft/TypeScript#5711 (comment)

[coveralls]

Coverage remained the same at 87.971% when pulling 58ae699 on alex-okrushko:master into 550b67e on ngrx:master.

[brandonroberts]

Will you remove the serializer commit from this PR since you have another one open for that?

[alex-okrushko]

Ah, yeah, not sure how that one got here as well...

[alex-okrushko]

@brandonroberts Done.