Enforcing type safety for custom Hash function

[drkameleon]

Update example code (saved as `hashy.nim`):
    
    
    import std/[tables, hashes]
    
    type
        Value = ref object
            i: int
    
    proc `==`*(a: Value, b: Value): bool =
        result = a.i == b.i
        echo "Used our =="
    
    proc hash*(a: Value): Hash =
      result = hashes.hash(a.i)
      echo "Used our hash for " & $(a.i)
    
    var a: Table[Value, Value]
    
    proc getIndex*(k: Value): int =
        if a.hasKey(k):
             return a[k].i
        else:
             return -1
    
    proc setIndex*(k: Value, v: int) =
        a[k] = Value(i: v)
    
    a[Value(i:100)] = Value(i:300)
    assert getIndex(Value(i:300)) == -1
    assert getIndex(Value(i:100)) == 300
    setIndex(Value(i:3), 400)
    assert getIndex(Value(i:3)) == 400
    
    
    Run

Compiled with `nim c hashy.nim`.

Running it gives me:
    
    
    Used our hash for 100
    Used our hash for 3
    
    
    Run

So, my conclusion is that our custom hash function is only getting called for 
direct index-assignments, e.g. `a[k] = X`