Add varargs to defines, defines as C imports, and CLI mangle option

[elcritch]

Hey! Thanks for keeping c2nim up and working!

I decided to take the time to fix/tweak a few things that I run into a lot and figure others might find them useful.

• Importing C #defines macro functions as external C functions using importc pragma
• Import C #defines constants as external C var's using importc
• Add VARARGS support for #define macro functions supporting templates
  • Note this doesn't create a proper macro to handle the varargs but it creates filler args: seq[untyped]
• Add VARARGS support for #define macro functions supporting C imports with {.varargs.} pragma
• Add command line option to pass mangle directives (useful for all those uint8_t types from <stdint.h>)

I'll work on adding the test cases I'm using tomorrow. I haven't run the test suite yet, but I'll do that tomorrow. However, I wanted to get this up so I wouldn't forget to PR it later.

Also, any feedback/changes would be great. I tried to follow the coding style as much as I could. Still I'm not entirely familiar with the correct PNode structures for some of the items. Particularly if there's a "proper" way to handle some of the constructs.

[Araq]

Looks very good, will review it in more detail later. Feel free to ping me again in order to remind me.

[elcritch]

Ok, nice. I used it on some bigger files and there is one issue so far. The "import as proc" seems to only spit out the first import if there's multiple definitions in a row without comments in between. That'll need fixed. I'll put it on my docket for next week.