Add some comments around various calls

Per review from workingjubilee

Author: ehuss

src/backtrace/dbghelp64.rs

@@ -87,6 +87,7 @@ pub unsafe fn trace(cb: &mut dyn FnMut(&super::Frame) -> bool) {
  use core::ptr;
 
  // Capture the initial context to start walking from.
+ // FIXME: shouldn't this have a Default impl?
  let mut context = unsafe { core::mem::zeroed::<MyContext>() };
  unsafe { RtlCaptureContext(&mut context.0) };
 

src/symbolize/gimli.rs

@@ -439,6 +439,7 @@ pub unsafe fn resolve(what: ResolveWhat<'_>, cb: &mut dyn FnMut(&super::Symbol))
  // Extend the lifetime of `sym` to `'static` since we are unfortunately
  // required to here, but it's only ever going out as a reference so no
  // reference to it should be persisted beyond this frame anyway.
+ // SAFETY: praying the above is correct
  let sym = unsafe { mem::transmute::<Symbol<'_>, Symbol<'static>>(sym) };
  (cb)(&super::Symbol { inner: sym });
  };

src/symbolize/gimli/libs_windows.rs

@@ -24,6 +24,7 @@ unsafe fn add_loaded_images(ret: &mut Vec<Library>) {
  return;
  }
 
+ // huge struct, probably should avoid manually initializing it even if we can
  let mut me = MaybeUninit::<MODULEENTRY32W>::zeroed().assume_init();
  me.dwSize = mem::size_of_val(&me) as u32;
  if Module32FirstW(snap, &mut me) == TRUE {