difficulties with const structs and alias this / template functions

[Dennis]

I'm making a fixed point numeric type and want it to work 
correctly with const. First problem:

```
const q16 a = 6;
a /= 2;          // compiles! despite `a` being const.
writeln(a);      // still 6
a.toQ32 /= 2;    // what's actually happening
```

My q16 type has an implicit conversion to q32 (like how int can 
be converted to long):
```
q32 toQ32() const {
   return q32(...);
}
alias toQ32 this;
```
How do I make it so that a const(q16) will be converted to a 
const(q32) instead of mutable q32?

Second problem:
```
Q log2(Q)(Q num) if (is(Q : q16) || is(Q : q32)) {
     import std.traits: Unqual;
     Unqual!Q x = num;
     // actual code
}
```
When I call this with a const(q16), Q is resolved to const(q16) 
so I have to unqualify Q every time. It works, but feels clumsy. 
Is there an easier way to automatically de-const parameters? 
We're working with small value types here, it should be simple.

If anyone knows any other pitfalls with const, I'd love to know 
them.