Emit a warning when x.ToString(), x.GetHashCode or x.Equals(y) are used directly on option-typed values

[@dsyme says] Proposal after discussion: emit a warning when x.ToString(), x.GetHashCode or x.Equals(y) are used directly on option-typed values, suggesting to use string or x = y instead. Or more generally when Equals, GetHashCode or ToString are used on F#-declared types known to have null as a representation.

### Description

None.ToString();;
None.GetHashCode();;
None.Equals(None);;

lead to NullReferenceException in fsi and fsc.

#### Known workarounds

Use ``string x``, ``hash x`` and ``x = y``