feat: re-factor tree_r_last building for external re-use

reduce conversions in encode (@dignifiedquire)
feat: reduce conversions for CPU tree building differently
feat: use an enum return type to avoid transmute (unsafe)
fix: properly use GPU if possible

storage-proofs-core/src/data.rs

@@ -100,6 +100,14 @@ impl<'a> Data<'a> {
         }
     }
 
+    pub fn empty() -> Self {
+        Data {
+            raw: None,
+            path: None,
+            len: 0,
+        }
+    }
+
     pub fn len(&self) -> usize {
         self.len
     }

storage-proofs-porep/src/encode.rs

@@ -2,13 +2,17 @@ use blstrs::Scalar as Fr;
 use filecoin_hashers::Domain;
 
 pub fn encode<T: Domain>(key: T, value: T) -> T {
-    let mut result: Fr = value.into();
-    let key: Fr = key.into();
+    let value: Fr = value.into();
+    let mut result: Fr = key.into();
 
-    result += key;
+    encode_fr(&mut result, value);
     result.into()
 }
 
+pub fn encode_fr(key: &mut Fr, value: Fr) {
+    *key += value;
+}
+
 pub fn decode<T: Domain>(key: T, value: T) -> T {
     let mut result: Fr = value.into();
     let key: Fr = key.into();

storage-proofs-porep/src/stacked/vanilla/mod.rs

@@ -29,4 +29,4 @@ pub use encoding_proof::EncodingProof;
 pub use graph::{StackedBucketGraph, StackedGraph, EXP_DEGREE};
 pub use labeling_proof::LabelingProof;
 pub use params::*;
-pub use proof::{StackedDrg, TOTAL_PARENTS};
+pub use proof::{StackedDrg, TreeRElementData, TOTAL_PARENTS};