Expose `ScriptBuf`'s `capacity`

There are already several methods referring to capacity but none to
retrieve it. Those methods also promise certain behavior that mandates
having *a* capacity field inside the struct, so no changes in layout
will ever remove it. Thus it's OK to expose the field.

Aside from exposing the field, this also fixes up the tests to obey the
sanity rules.

primitives/src/script/owned.rs

@@ -91,6 +91,14 @@ impl ScriptBuf {
     /// Panics if the new capacity exceeds `isize::MAX bytes`.
     #[inline]
     pub fn reserve_exact(&mut self, additional_len: usize) { self.0.reserve_exact(additional_len); }
+
+    /// Returns the number of **bytes** available for writing without reallocation.
+    ///
+    /// It is guaranteed that `script.capacity() >= script.len()` always holds.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.0.capacity()
+    }
 }
 
 impl Deref for ScriptBuf {
@@ -161,20 +169,20 @@ mod tests {
     #[test]
     fn script_buf_capacity() {
         let script = ScriptBuf::with_capacity(10);
-        assert!(script.0.capacity() >= 10);
+        assert!(script.capacity() >= 10);
     }
 
     #[test]
     fn script_buf_reserve() {
         let mut script = ScriptBuf::new();
         script.reserve(10);
-        assert!(script.0.capacity() >= 10);
+        assert!(script.capacity() >= 10);
     }
 
     #[test]
     fn script_buf_reserve_exact() {
         let mut script = ScriptBuf::new();
         script.reserve_exact(10);
-        assert!(script.0.capacity() >= 10);
+        assert!(script.capacity() >= 10);
     }
 }