misc: drop a few low-hanging unsafes

Remove a couple of unnecessary unsafes:
- The NonZeroUsize is a constant where the unwrap will optimize away
anyway and we don't have an unsafe without any good reason there :)
- The other two were simply not needed, lifetimes worked fine, maybe
Rust became better since that code was written? NLL? Anyway, they're
gone now

cli/src/commands/run.rs

@@ -63,7 +63,7 @@ pub fn cmd_run(ui: &mut Ui, command: &CommandHelper, args: &RunArgs) -> Result<(
         // SAFETY:
         // We use a internal constant of 4 threads, if it fails
         let available = std::thread::available_parallelism()
-            .unwrap_or(unsafe { NonZeroUsize::new_unchecked(4) });
+            .unwrap_or(NonZeroUsize::new(4).unwrap());
         available.into()
     };
     Err(user_error("This is a stub, do not use"))

lib/src/default_index_store.rs

@@ -839,11 +839,8 @@ impl<'a> CompositeIndex<'a> {
         if pos.0 >= num_parent_commits {
             self.0.segment_entry_by_pos(pos, pos.0 - num_parent_commits)
         } else {
-            let parent_file: &ReadonlyIndexImpl = self.0.segment_parent_file().unwrap().as_ref();
-            // The parent ReadonlyIndex outlives the child
-            let parent_file: &'a ReadonlyIndexImpl = unsafe { std::mem::transmute(parent_file) };
-
-            CompositeIndex(parent_file).entry_by_pos(pos)
+            let parent_file = self.0.segment_parent_file().unwrap();
+            CompositeIndex(&**parent_file).entry_by_pos(pos)
         }
     }
 

lib/src/stacked_table.rs

@@ -51,14 +51,8 @@ pub trait TableSegment {
     }
 
     fn get_value<'a>(&'a self, key: &[u8]) -> Option<&'a [u8]> {
-        if let Some(value) = self.segment_get_value(key) {
-            return Some(value);
-        }
-        let parent_file = self.segment_parent_file()?;
-        let parent_file: &ReadonlyTable = parent_file.as_ref();
-        // The parent ReadonlyIndex outlives the child
-        let parent_file: &'a ReadonlyTable = unsafe { std::mem::transmute(parent_file) };
-        parent_file.get_value(key)
+        self.segment_get_value(key)
+            .or_else(|| self.segment_parent_file()?.get_value(key))
     }
 }