Make Manifest and Package easier to instantiate

[Turbo87]

Manifest and Package are currently quite annoying to instantiate because all the fields need to be explicitly assigned even if only a small subset is used.

This PR implements the Default trait for the Manifest struct allowing an instantiation like Manifest { package, ..Default::default() }.

This PR also adds a new() fn to the Package struct for similar reasons. An alternative to this is implementing Default too, but that would leave us with name: "" and version: "". I'm undecided which way feels better, but for new() there is prior art.

Finally, this PR also adds a small serialization test case, to ensure that these API keep working in the future :)

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