modify the file header part of aggsig code

[garyyu]

Corresponding to mimblewimble/secp256k1-zkp#29

There're also 2 files in this repo:

• aggsig.rs
• pedersen.rs

I guess there's no any upstream for these 2 source files. Could you please confirm this? @yeastplume

I also suggest to change the header part, currently it's:

// Bitcoin secp256k1 bindings
// Written in 2014 by
//   Dawid Ciężarkiewicz
//   Andrew Poelstra
//

And one more suggestion: Could we do rustfmt on these 2 files? since there's no upstream for these 2 files and no need to worry merging.

[yeastplume]

okay, done