Safe `enum` conversion

[ElegantBeef]

Stealing a macro from the `std/setutils` in devel, got an _ok_ implementation 
for holey and non-holey enums: 
    
    
    import std/macros
    type
      X = enum
        Z
    
    type
      HoleyEnum = enum
        AVal = 3
        BVal = 5
    
    macro enumElementsAsSet(enm: typed): untyped = result = 
newNimNode(nnkCurly).add(enm.getType[1][1..^1])
    
    proc toEnum*(val: SomeInteger, E: typedesc[enum]): E =
      const enmRange = E.low.ord .. E.high.ord
      when E is Ordinal:
        if val in enmRange:
          E(val)
        else:
          raise (ref ValueError)(msg: $val & " cannot be converted to the enum: 
" & $E)
      else:
        if val in enmRange and val.E in E.enumElementsAsSet:
          E(val)
        else:
          raise (ref ValueError)(msg: $val & " cannot be converted to the enum: 
" & $E)
    
    var a = 42
    
    let b = a.toEnum(X)
    let c = 4.toEnum(HoleyEnum)
    
    
    Run