Release a major update with nested-values and dynamic-keys in core instead of features

[MOZGIII]

I figured having those hidden behind feature flags does more bad than good: the features are poorly supported and cause runtime panics from the surrounding infrastructure.
This fractures the ecosystem in a way that when one can attempts using a package it can compile and have all interfaces check out, but panic at runtime with something that should've definitely been a compile error not only in rust, but in any modern type-safe language.

[Techcable]

Enabling nested-values and serde support by default was part of the hypothetical changes mentioned for slog v3

I would consider these feature flags experimental features for now. I agree these should eventually be the defaults, but for now we're not really in a hurry to upgrade to v3