These changes remove the JSON encoding/decoding steps that are
performed during the serialization of ini and dotenv files. This
roundtrip loses type information during the transformation which
causes values to be incorrectly converted to the JSON marshaller
defaults (int becomes float64, bool becomes string, etc, etc). In
place of this JSON encoding, the mapstructure library allows for a
direct conversion between the Metadata struct and
map[string]interface{} needed to leverage the stores.Flatten and
stores.Unflatten functions.

In addition this adds mapstructure tags to the metadata structures to
allow backwards compatibility with the JSON encoding.

Resolves getsops#879 & resolves getsops#857

This is a re-submission of @acastle's getsops#1009 with mapstructure updated
to v1.5.0 and unused imports removed applied to getsops/sops/main HEAD.

Restore WeakDecode to UnflattenMetadata.

Remove unused privateKeySizeLimit.