Use mapstructure to (de-)serialize internal metadata; move metadata processing from specific stores to general place

[felixfontein]

Right now, metadata is handled quite strangely when flattened (by going through JSON), and all stores somehow have to deal with metadata. This PR cleans this up as follows:

• Use mapstructure to convert to/from metadata and coerce types.
• Convert metadata to/from sops.TreeBranch, and provide three options for handling flattening (none; keep sops toplevel map; flatten completely).
• Properly handle errors during flattening and unflattening. Right now you can panic SOPS and overwrite values by playing around with flattened metadata.
• Remove old public APIs for dealing with internal metadata and (un-)flattening.

While this is a breaking change, I think this is still OK:

• The APIs that are broken are internal ones. To from our promise, they're not public and it's not a breaking change.
• They should only be used by store implementations, which are all part of SOPS itself. This means that even though some other programs use parts of SOPS as a library that aren't public, it is unlikely that they directly use these parts of SOPS that are changed by this PR.

This is related to #1401, #1338, #1046, #1009, which were earlier attempts to introduce mapstructure.