if __ctfe and __ctfe functions

[Quirin Schroll]

This was suggested before, but I’ll try to show another angle of 
this.

### The `if __ctfe` construct

It would be a new syntactic construct different from `if 
(__ctfe)`. We could also decide to special-case `if (__ctfe)`, 
either way: The branch in which `__ctfe` is guaranteed to be true 
should be ignored w.r.t. some attribute checks: In a `__ctfe` 
section,
- a `@nogc` function can allocate, and
- a `@safe` function can do pointer arithmetic.
- *maybe:* a `nothrow` function can throw, except when 
control-flow reaches the `throw` expression, it halts for error.

Allocation at CTFE isn’t what `@nogc` is about and safety checks 
are done at CTFE anyways even for runtime-unsafe operations.

### The function attribute `__ctfe`

- Add `__ctfe` as a function attribute.
- Overloading on `__ctfe` is possible and useful: Overload 
resolution for run-time code generation discards all `__ctfe` 
functions; overload resolution at CTFE prefers an overload if, 
after partial ordering, it is the only overload annotated 
`__ctfe` among the best matches.
- A `__ctfe` function admits no safety attributes or `@nogc` 
(make no sense).
- A `__ctfe` function is guaranteed not to be emitted into the 
binary.