minimize heap allocations in core data structures

[sdemos]

The Data type, which holds the representations for all the supported key types, currently stores all the data in some number of Vec<u8>s. It should be converted to use arrays instead.

[sdemos]

For those following along at home, this issue (and all of the subsequent issues I also filed), are things that I only have a vague idea of how to do, or if they are even possible/reasonable. I just wanted to write them down so they are more organized and I can come back to them later.