tamer: span::Span::offset_add: Make const

This behavior is unchanged, but it allows us to create more constant spans
for testing.  For example:

  const S = DUMMY_SPAN.offset_add(1).unwrap();

This, in turn, will allow for removing lazy_static! for tests that use it
for span generation.

DEV-10863

tamer/src/span.rs

@@ -313,10 +313,14 @@ impl Span {
     ///   provided value.
     /// If the resulting offset exceeds [`global::SourceFileSize`],
     ///   the result will be [`None`].
-    pub fn offset_add(self, value: global::SourceFileSize) -> Option<Self> {
-        self.offset
-            .checked_add(value)
-            .map(|offset| Self { offset, ..self })
+    pub const fn offset_add(
+        self,
+        value: global::SourceFileSize,
+    ) -> Option<Self> {
+        match self.offset.checked_add(value) {
+            Some(offset) => Some(Self { offset, ..self }),
+            None => None,
+        }
     }
 }