* make destructors accept non var parameters
* define nimAllowNonVarDestructor
* add a test case and a changelog
* update documentation and error messages
* deprecate destructors taking 'var T'
* make more standard libraries work with `nimPreviewSlimSystem`
* typo
* part two
* Delete specutils.nim
* fixes more tests
* more fixes
* fixes tests
* fixes three more tests
* add formatfloat import
* fix
* last
`Task.callback` cannot be nil, we need to raise it at debug and release mode
Situations:
- if users create a Task object without using `toTask` and invoke the Task
- if users already move the Task and invoke the Task