feat(source): use `usize` for length (#265)

BREAKING CHANGE: This changes `SourceSpan`'s length type to `usize`.

src/protocol.rs

@@ -526,10 +526,10 @@ pub struct SourceSpan {
 
 impl SourceSpan {
     /// Create a new [`SourceSpan`].
-    pub const fn new(start: SourceOffset, length: SourceOffset) -> Self {
+    pub const fn new(start: SourceOffset, length: usize) -> Self {
         Self {
             offset: start,
-            length: length.offset(),
+            length,
         }
     }
 
@@ -559,8 +559,8 @@ impl From<(ByteOffset, usize)> for SourceSpan {
     }
 }
 
-impl From<(SourceOffset, SourceOffset)> for SourceSpan {
-    fn from((start, len): (SourceOffset, SourceOffset)) -> Self {
+impl From<(SourceOffset, usize)> for SourceSpan {
+    fn from((start, len): (SourceOffset, usize)) -> Self {
         Self::new(start, len)
     }
 }

tests/derive.rs

@@ -406,7 +406,7 @@ impl ForwardsTo {
     fn new() -> Self {
         ForwardsTo {
             src: SNIPPET_TEXT.into(),
-            label: SourceSpan::new(11.into(), 6.into()),
+            label: SourceSpan::new(11.into(), 6),
         }
     }
 }