Deserializing ints of arbitrary size or signedness

[ExpHP]

Even though I decided against automatic widening conversions due to implementation complexity, we could still have types that allow deserializing multiple different choices for signedness or size:

// these wrappers will implement Deserialize, but not Serialize.
pub struct Int(pub i128);
pub struct SignedInt(pub i64);
pub struct UnsignedInt(pub u64);

This is important to have because, for instance, sparse array .npz files automatically switch between using <i4 and <i8 based on the data.

[ExpHP]

Orrrrrrrrrrrr we could just support widening in spite of the complexity.

(Damn. I just cleaned up that macro!)