Allow schema from variable name in @row

[ericphanson]

I've been finding it helpful to add write_table-type functions to automatically add a project-specific schema, and therefore would like to do things like:

const MySchema = Legolas.Schema("my-schema@1")
const MyRow = Legolas.@row(MySchema, a::Int)

function write_table(path_or_io, table; kwargs...)
    Legolas.write(path_or_io, table, MySchema; kwargs...)
end

However, Legolas.@row requires a hard-coded schema name there, not the MySchema variable. It would be nice if this could be generalized. I'm not sure if this is even possible because I don't know think the value of MySchema is available at macro-expansion time. But it is a pain point so I figured I'd open an issue for discussion.

[jrevels]

Hmm I see what you mean.

I don't think I hit this whenever I work with Schemas since I generally prefer "spelling" a-priori-known Schema values as literals instead of rebinding them to a const.

That being said, I'm not opposed to supporting something like this if the implementation can be sussed out

[jrevels]

I think this can be closed after #54