rvalue references

[Diggory]

Based on DIP25/35 but slightly simplified, these rules would make 
"ref" safe, would minimise code breakage, and only marginally 
restricts the usage of "ref". The notion of "lifetime" is purely 
a tool for simplifying the explanation of the rules.

Lifetimes
0 = temporary/rvalue
1 = local variable/non-ref parameter/scope-ref parameter
2 = ref parameter
3 = static/heap allocated value

A value with any lifetime may be passed as any parameter into a 
function.

The & operator is safe only for lifetime >= 3, in other cases it 
is unsafe or system

Only values with lifetime >= 2 can be returned by ref

The lifetime of a return value is determined by the caller as 
follows:
     - Take the lifetimes of all values passed in using "ref" 
without the "scope" qualifier
     - The lifetime of the return value is the minimum lifetime of 
these values

OPTIONAL:
- A return value marked as "out" is guaranteed to have a lifetime 
 >= 3 (equivalent to all ref parameters being marked "scope")
- Marking a method as "scope" is equivalent to marking the "this" 
parameter as such