manifest: remove manifest.add and add memmfctx.write This removes one more dependency on the manifest class by moving the write functionality onto the memmanifestctx classes and changing the one consumer to use the new API. By moving the write path to a manifestctx, we now give the individual manifests control over how they're read and serialized. This will be useful in developing new manifest formats and storage systems.

context: add manifestctx property on changectx This allows us to access the manifestctx for a given commit. This will be used in a later patch to be able to copy the manifestctx when we want to make a new commit.