Compile basepoint multiplication tables into a binary form in build.rs

[LLFourn]

Right now we compile the multiplication tables into rust code as in the original parity code:
https://github.com/LLFourn/secp256kfun/blob/master/secp256kfun/build.rs#L41

We should compile it to a binary form and then read it in efficiently at compile time. We will probably need const_looping as mentioned in #3 (which will be stabilised soon I think).

[LLFourn]

This was almost possible but rust-lang/rust#73083 got in the way.

[LLFourn]

We're moving away from the parity backend so this is no longer relevant.