feat: oci tar format, bit-perfect disk storage for config and manifest, concurrent image pulls (#88)

* oci: retain bit-perfect copies of manifest and config on disk

* feat: oci tar format support

* feat: concurrent image pulls

crates/oci/src/schema.rs

@@ -0,0 +1,29 @@
+use std::fmt::Debug;
+
+#[derive(Clone, Debug)]
+pub struct OciSchema<T: Clone + Debug> {
+    raw: Vec<u8>,
+    item: T,
+}
+
+impl<T: Clone + Debug> OciSchema<T> {
+    pub fn new(raw: Vec<u8>, item: T) -> OciSchema<T> {
+        OciSchema { raw, item }
+    }
+
+    pub fn raw(&self) -> &[u8] {
+        &self.raw
+    }
+
+    pub fn item(&self) -> &T {
+        &self.item
+    }
+
+    pub fn into_raw(self) -> Vec<u8> {
+        self.raw
+    }
+
+    pub fn into_item(self) -> T {
+        self.item
+    }
+}