Add ImportC compiler to dmd

[Steven Schveighoffer]

On 5/11/21 10:34 AM, Steven Schveighoffer wrote:
> I had this idea: what if in D code, you can access CPP macros 
> explicitly. Like:
> 
> #CMACRO(<macro expression)

Hm... macro is still a reserved keyword, so we could use that also.

-Steve