v0.5.7.0

Result<T, E> overhaul.

Background

• The main use-case of Result<T, E> is to serve as return of a function that can either have a successful result of
  type T or an error of type E.
• In pretty much all cases the error is just a string, i.e. the common use is Result<T, string>

Changes

• For simplification the type parameter for the error case is dropped, i.e. a result is just Result<T>
  • The old notation Result<T, E> (or rather Result<T, string>) will still work, but the second parameter is ignored
• The result.error field is now a newly introduced core::error::Error
  • For compatibility core::error::Error will be automatically converted to a string
  • Just like before an error Result can be created using the builtin Err function with an error message
• In addition to the error message the core::error::Error additional contains a stack_trace to help locating
  where the Error was created
• As an additional helper there this a core::error::current_stack() function to obtain the current stack trace for
  debugging purposes.