Updating secp256k1 and supporting full no-std features

[elichai]

Now that both bitcoin-core/secp256k1#566 and bitcoin-core/secp256k1#595 Are merged,

I would like to update the code in depend to secp256k1 master and start working on integrating both of these features into the library here.

I'm not sure how the preallocation will look like i'll know more after i'll dive into the C code and try and see what's possible in rust, but if there are no bounds to that memory at all the user of this library might need to provide the buffer.

i'll start working on this this weekend. just a heads up in case anyone else thought of doing this :)

Any comments/ideas are welcome :)

[elichai]

What do you guys think about using https://github.com/rust-lang/rust-bindgen to generate the rust definitions?