Fix some clippy warnings

src/blob.rs

 /// The decoded content of a blob (analogous to `BlobType`).
 pub enum BlobDecode<'a> {
     /// Blob contains a `HeaderBlock`.
-    OsmHeader(HeaderBlock),
+    OsmHeader(Box<HeaderBlock>),
     /// Blob contains a `PrimitiveBlock`.
     OsmData(PrimitiveBlock),
     /// An unknown blob type with the given string identifier.
     pub fn decode(&self) -> Result<BlobDecode> {
         match self.get_type() {
             BlobType::OsmHeader => {
-                self.to_headerblock()
-                    .map(BlobDecode::OsmHeader)
+                let block = Box::new(self.to_headerblock()?);
+                Ok(BlobDecode::OsmHeader(block))
+
             },
             BlobType::OsmData => {
-                self.to_primitiveblock()
-                    .map(BlobDecode::OsmData)
+                let block = self.to_primitiveblock()?;
+                Ok(BlobDecode::OsmData(block))
             },
             BlobType::Unknown(x) => Ok(BlobDecode::Unknown(x)),
         }

src/mmap_blob.rs

             .chain_err(|| "failed to parse Blob")?;
         match self.header.get_field_type() {
             "OSMHeader" => {
-                let block: osmformat::HeaderBlock = decode_blob(&blob).unwrap();
-                Ok(BlobDecode::OsmHeader(HeaderBlock::new(block)))
+                let block = Box::new(HeaderBlock::new(decode_blob(&blob)?));
+                Ok(BlobDecode::OsmHeader(block))
             }
             "OSMData" => {
-                let block: osmformat::PrimitiveBlock = decode_blob(&blob).unwrap();
+                let block: osmformat::PrimitiveBlock = decode_blob(&blob)?;
                 Ok(BlobDecode::OsmData(PrimitiveBlock::new(block)))
             }
             x => Ok(BlobDecode::Unknown(x)),
             Ok(x) => x,
             Err(e) => {
                 self.last_blob_ok = false;
-                return Some(Err(e.into()));
+                return Some(Err(e));
             },
         };