Use "Size" field to describe blobs over "Length"

After consideration, we've changed the main descriptor field name to for number
of bytes to "size" to match convention. While this may be a subjective
argument, commonly we refer to files by their "size" rather than their
"length". This will match other conventions, like `(FileInfo).Size()` and
methods on `io.SizeReaderAt`. Under more broad analysis, this argument doesn't
necessarily hold up. If anything, "size" is shorter than "length".

Signed-off-by: Stephen J Day <stephen.day@docker.com>

blobs.go

@@ -49,8 +49,8 @@ type Descriptor struct {
 	// encoded as utf-8.
 	MediaType string `json:"mediaType,omitempty"`
 
-	// Length in bytes of content.
-	Length int64 `json:"length,omitempty"`
+	// Size in bytes of content.
+	Size int64 `json:"size,omitempty"`
 
 	// Digest uniquely identifies the content. A byte stream can be verified
 	// against against this digest.