Add support for half-precision floats in binary 1.1 reader.

[nirosys]

From Matt's comment on 756.

Can we create an issue to track this? It would be great for users (and our own benchmarks) if we could use 2-byte floats for "common" numbers like 1.0.

We'll probably need to settle on an f16 implementation (such as half).

[zslayton]

In the medium term, Rust itself is getting native support for an f16 type (and f128, but we can't use that). The RFC was merged, but the implementation is still in-flight.