Add Rayon support

The code is adapted from `rayon-hash`.

Author: hcpl

Cargo.toml

@@ -10,14 +10,19 @@ keywords = ["hash", "no_std", "hashmap", "swisstable"]
 categories = ["data-structures", "no-std"]
 
 [dependencies]
-scopeguard = { version = "0.3", default-features = false }
 byteorder = { version = "1.0", default-features = false }
+scopeguard = { version = "0.3", default-features = false }
+
+# For external trait impls
+rayon = { version = "1.0", optional = true }
 serde = { version = "1.0", default-features = false, optional = true }
 
 [dev-dependencies]
+lazy_static = "~1.2"
+rand = "0.5.1"
+rayon = "1.0"
 rustc-hash = "1.0"
 serde_test = "1.0"
-rand = "0.5.1"
 
 [features]
 nightly = []

src/external_trait_impls/mod.rs

@@ -0,0 +1,4 @@
+#[cfg(feature = "rayon")]
+pub(crate) mod rayon;
+#[cfg(feature = "serde")]
+mod serde;

src/external_trait_impls/rayon/helpers.rs

@@ -0,0 +1,25 @@
+use alloc::collections::LinkedList;
+use alloc::vec::Vec;
+
+use rayon::iter::{IntoParallelIterator, ParallelIterator};
+
+
+/// Helper for collecting parallel iterators to an intermediary
+pub(super) fn collect<I: IntoParallelIterator>(iter: I) -> (LinkedList<Vec<I::Item>>, usize) {
+    let list = iter
+        .into_par_iter()
+        .fold(Vec::new, |mut vec, elem| {
+            vec.push(elem);
+            vec
+        }).map(|vec| {
+            let mut list = LinkedList::new();
+            list.push_back(vec);
+            list
+        }).reduce(LinkedList::new, |mut list1, mut list2| {
+            list1.append(&mut list2);
+            list1
+        });
+
+    let len = list.iter().map(Vec::len).sum();
+    (list, len)
+}