Allow typedesc assignments with let/var at "compile time".

[deech]

Currently typedesc assignments are only allowed with const variables, but they should also be allowed with let/var so long as they are in a static/const/when context:

const C = 
  block:
    let t : typedesc[int] = int
    ...

when(
  block:
    let t : typedesc[int] = int
    true
):
  ...

static:
  let t : typedesc[int] = int
  ...

[mratsim]

Shouldn't we use type t: type int = int anyway?

[deech]

I had no idea that syntax existed.

[krux02]

yes use type definitions please.