Feature flag out the external dependencies #378
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The rationale for this change is that there are cases where people would like to use the general validation infrastructure, as well as the validation derive macros for simple data types, but do not want to pull in these extra dependencies to their dependency graph. The remaining validators that are not affected by this PR affect built-in types and as such are always enabled.
By enabling the new feature flags by default, this is not a breaking change, unless the downstream user explicitly set default-features = false.