Preserve comments isolated in documents (and an error decrypting in edge cases)

When testing comments at the top of YAML files in #757, I found that comments in YAML streams which are isolated into their own documents are coalesced into the adjacent document (but only if they are at the very top of the YAML stream). Also, I found a possible bug where sops will encrypt a document that it cannot decrypt later on whenever these isolated comments are located anywhere else in the YAML stream.

Coalescing separate documents into a single document:

#@data/values
---
foo: bar

Becomes:

#@data/values
foo: bar

Errors: Each of the following variants of the above YAML stream will encrypt without an error. However, they cannot be decrypted, resulting in the exact same error message:

Could not marshal tree: Error marshaling to yaml: yaml: expected SCALAR, SEQUENCE-START, MAPPING-START, or ALIAS, but got document end

---
#@data/values
---
foo: bar

foo: bar
---
#@data/values
---
baz: qux

foo: bar
---
#@data/values

Note: While these examples utilize some code from ytt which handles comments semantically, it's not expected for sops to support ytt syntax per se. Rather, it's just a real-world example of the usefulness of having comments in this positions and isolated in this way, particularly since sops can be used to encrypt values that would be used in a templating engine like ytt (for annotations in a document containing sensitive data that are then interpreted by a parser).

Reproduce the listed YAML streams with sops, checking both encryption and decryption behavior. Trace how isolated comments and document boundaries are parsed and marshaled; done means comments remain in their own documents and each example can be decrypted successfully.