Is there a way to ensure that a viriable is "passed by address" (or reference) in Nim

[cblake]

@jibal I just copied the pre-existing `mget` interface to do something more 
useful. That said, another way to avoid unnecessary object construction 
compatible with the existing interface is: 
    
    
    const iniOb = whatever
    ...in some proc or loop:
      mgetOrPut(key, iniOb).doSomething
    
    
    Run

That way the generic assign copy of the `val` is only done on `rawInsert` after 
the key is known to be absent (in the OrPut branch). The `const` could also be 
a `let` or just before a loop to do many, etc.