Miscellaneous code cleanup

Miscellaneous code cleanup

- Remove some explicit ctors
- Remove some unnecessary casts
- Mark some fields readonly
- Follow style guidelines for visibility/static ordering in signatures
- Move usings to top of file
- Delete some stale comments
- Added names for bool args at call sites
- Use expression-bodied members in a few places
- Pass lower bounds to Array.Copy
- Remove unnecessary "success" local in QueueUserworkItemHelper

Commit migrated from https://github.com/dotnet/coreclr/commit/ade717c7bbca40632db3b42885589909d645fc1a