feat: write contributions file

The input file is split into 3 pieces, `*.vk`, `*.params` and `*.contribs`.
In order to verify they are actually the pieces from the original file,
you can concatenate them to get the orginal file back via:

```
cat *.vk *.params *.contribs > same-as-original.bin
```

filecoin-proofs/src/bin/phase2.rs

@@ -30,7 +30,7 @@ use simplelog::{self, CombinedLogger, LevelFilter, TermLogger, TerminalMode, Wri
 use storage_proofs::compound_proof::{self, CompoundProof};
 use storage_proofs::hasher::Sha256Hasher;
 use storage_proofs::merkle::MerkleTreeTrait;
-use storage_proofs::parameter_cache::CacheableParameters;
+use storage_proofs::parameter_cache::{self, CacheableParameters};
 use storage_proofs::porep::stacked::{StackedCircuit, StackedCompound, StackedDrg};
 use storage_proofs::post::fallback::{FallbackPoSt, FallbackPoStCircuit, FallbackPoStCompound};
 
@@ -1535,8 +1535,26 @@ fn main() {
                         - 64
                         - 4
                         - (param_num as u64 * 544);
-                    println!("size of the params file is {} bytes", params_file_size);
-                    io::copy(&mut input_file.take(params_file_size), &mut params_file).unwrap();
+                    println!("size of the parameters file is {} bytes", params_file_size);
+                    io::copy(
+                        &mut Read::by_ref(&mut input_file).take(params_file_size),
+                        &mut params_file,
+                    )
+                    .unwrap();
+                }
+
+                {
+                    // Writing the contributions to disk is not needed for the final parameters,
+                    // they won't be published, they are only there for verification purpose
+                    let contribs_path =
+                        format!("v{}-{}.contribs", parameter_cache::VERSION, &identifier);
+                    println!("writing contributions to file: {}", contribs_path);
+                    let mut contribs_file = File::create(&contribs_path).unwrap();
+                    let contribs_file_size = io::copy(&mut input_file, &mut contribs_file).unwrap();
+                    println!(
+                        "size of the contributions file is {} bytes",
+                        contribs_file_size
+                    );
                 }
 
                 {