Block import and export (#272)

* Block export and import

* Export and import using std streams

* Made AuthorituId::from_slice private

substrate/substrate/codec/src/slicable.rs

@@ -38,6 +38,7 @@ pub trait Input {
 	}
 }
 
+#[cfg(not(feature = "std"))]
 impl<'a> Input for &'a [u8] {
 	fn read(&mut self, into: &mut [u8]) -> usize {
 		let len = ::core::cmp::min(into.len(), self.len());
@@ -47,6 +48,13 @@ impl<'a> Input for &'a [u8] {
 	}
 }
 
+#[cfg(feature = "std")]
+impl<R: ::std::io::Read> Input for R {
+	fn read(&mut self, into: &mut [u8]) -> usize {
+		(self as &mut ::std::io::Read).read(into).unwrap_or(0)
+	}
+}
+
 /// Trait that allows zero-copy read/write of value-references to/from slices in LE format.
 pub trait Slicable: Sized {
 	/// Attempt to deserialise the value from input.