VM: allow setting ptr/pointer fields at CT

[timotheecour]

/cc @Araq

before PR:
Error: unhandled exception: intVal [FieldError]
whenever setting a ptr or pointer field (assigning a ptr variable at CT already works)

after PR:
setting a ptr or pointer field works

examples

there are many use cases, here's a simple one (example1 with pointer field) and a more elaborate one (example2 with ptr field)
example below requires building nim with -d:nimHasLibFFI and compiling with --experimental:compiletimeFFI for importc at CT

nim c -d:timn_D20191206T095533 --app:lib --outdir:$build_D/lib/ t0816b.nim # only needed for example2
nim c -r --experimental:compiletimeFFI t0816.nim 

t0816.nim:

import system/ansi_c
import ./t0816b

proc c_malloc*(size: csize_t): pointer {.importc: "malloc", header: "<stdlib.h>".}
proc c_fprintf*(f: CFilePtr, frmt: cstring): cint {.importc: "fprintf", header: "<stdio.h>", varargs, discardable.}

type Foo = object
  a: pointer
  b: CFilePtr

proc main()=
  var f: Foo

  ## example 1: pointer
  f.a = c_malloc(1234)

  ## example 2: ptr (CFilePtr = ptr CFile)
  f.b = getStderr()
  # c_fprintf(cstderr, "hello world\n") # doesn't work yet, so we need an intermediate library, see t0816b.nim
  c_fprintf(getStderr(), "hello world 1\n")
  c_fprintf(f.b, "hello world 2\n")
  ## re-assignment works
  f.b = getStdout()
  c_fprintf(f.b, "hello world 3\n")
  ## re-assignment to nil works
  f.b = nil

static: main()
main()

t0816b.nim:

when defined(timn_D20191206T095533):
  {.emit:"""
  #include <stdlib.h>
  #include <stdio.h>
  NIM_EXTERNC
  FILE* getStderr(){ return stderr;}
  FILE* getStdout(){ return stdout;}
  """.}

else:
  from system/ansi_c import CFilePtr
  const libF = "libt0816b.dylib"
  {.push importc, dynlib: libF.}
  proc getStderr*(): CFilePtr
  proc getStdout*(): CFilePtr
  {.pop.}

note

I added a comment regarding nkPtrLit, which is what compiler/evalffi.nim uses:

const
  nkPtrLit = nkIntLit # hopefully we can get rid of this hack soon

ideally, nkPtrLit could be a new member of TNodeKind to avoid conflation; but this could be done in future PR