Trait bounds inconsistent for metadata retrieval

[molpopgen]

Functions to add metadata look like this:

    pub fn add_mutation_with_metadata<
        S: Into<SiteId>,
        N: Into<NodeId>,
        M: Into<MutationId>,
        MD: MutationMetadata,
        T: Into<Time>,
    >(...)

But functions to retrieve the metadata look like:

    pub fn metadata<T: metadata::MetadataRoundtrip>(
        &'a self,
        row: MutationId,
    );

Thus, the trait bound on the retrieval function is less strict.

Fixing this would technically be a breaking change.
But in practice, no code would break because you cannot record metadata
of a different type (trait bound) in the first place.

If nothing else, we should fix it for cleanliness' sake.

[molpopgen]

Closed via #370