feat: Unsafe transmute for PDB symbols (#258)

This works around compilation errors on nightly when casting to the
generic `DynIterator` type. This is caused by invariant lifetimes on
PDB's stream type. The transmute is actually safe since the lifetime of
the data is proved to be larger than the lifetime of the derived object.

debuginfo/src/object.rs

@@ -346,7 +346,7 @@ impl<'d> ObjectLike for Object<'d> {
     }
 
     fn symbols(&self) -> DynIterator<'_, Symbol<'_>> {
-        Box::new(self.symbols())
+        unsafe { std::mem::transmute(Box::new(self.symbols()) as DynIterator<'_, _>) }
     }
 
     fn has_debug_info(&self) -> bool {

debuginfo/src/pdb.rs

@@ -292,7 +292,8 @@ impl<'d> ObjectLike for PdbObject<'d> {
     }
 
     fn symbols(&self) -> DynIterator<'_, Symbol<'_>> {
-        Box::new(self.symbols())
+        // TODO: Avoid this transmute by introducing explicit lifetimes on the trait.
+        unsafe { std::mem::transmute(Box::new(self.symbols()) as DynIterator<'_, _>) }
     }
 
     fn symbol_map(&self) -> SymbolMap<'_> {