Add bfloat16 data type

[jbms]

[jstriebel]

I would have assumed that this would be a separate spec under data-types, since it's optional.

I can see that this is a lot of boilerplate for a rather simple data type, but on the other hand not all future optional data types should be in the core spec IMO. So I'd draw the line at optional data-types.

[MSanKeys963]

Thanks for sending the PR, @jbms and reviewing @jstriebel.

@jbms - do you think bfloat16 should be under data-types?