Overload for seq<'t>

[gusty]

We can try adding an overload for seq<'t>, the first question to answer is whether it makes sense.

I think it could be problematic if it accepts all IEnumerable<'t> subclasses, for instance it will fail the roundtrip, since it will convert back to an IEnumerable<'t> of whichever underlying implementation we decide as default.

If we manage to make it strict to seq<'t> we just need to find out what's the best underlying implementation to use as default (an array, a list, a json reader), then the roundtrip will succeed in terms of types, it would just change the underlying representation which I don't think is really an issue, since the contract is about types.