[DO NOT MERGE] experiment with -d:nimImplicitCompleteStruct showing we need completeStruct instead of incompleteStruct

[timotheecour]

As I explained in proposal 2 from nim-lang/RFCs#205, {.incompleteStruct.} (introduced in 2012) should be deprecated, as it's assuming the wrong default (which is implicit assumption that importc types would be complete unless marked with {.incompleteStruct.}). Which is why the correct pragma should be {.completeStruct.} (as introduced in nim-lang#13926) and makes the opposite assumption that importc types are incomplete unless marked with {.completeStruct.}

this experiment-only PR introduces -d:nimImplicitCompleteStruct, which when enabled, allows CT sizeof; it then compares with c_sizeof. It shows that most importc types are in fact incomplete, and the CT sizeof differs from c_sizeof.
Only very few types are annotated with {.incompleteStruct.} so most code out there would be wrong if that flag had any meaning.
It would be pointless to have to annotate all those types with {.incompleteStruct.}, when in fact the sane thing to do is to opt-in CT sizeof instead of opt-out.

nim c -r --stacktrace:off -d:nimImplicitCompleteStruct -f $timn_D/src/timn/exp/abichecks.nim

prints:

Dirent struct dirent => 272 vs 1048
Tflock struct flock => 32 vs 24
Glob glob_t => 24 vs 88
Group struct group => 24 vs 32
Ipc_perm struct ipc_perm => 20 vs 24
Stat struct stat => 104 vs 144
Statvfs struct statvfs => 88 vs 64

{.emit:"""
#define c_astToStrImpl(T) #T
"""
.}

import macros
import strutils
import unittest

import posix

template ensureCgen(T: typedesc) =
  ## ensures cgen
  var a {.volatile.}: T

template c_astToStr(T: typedesc): string =
  block:
    # proc needed pending https://github.com/nim-lang/Nim/issues/13943 D20200409T215527
    proc fun2(): string =
      var s: cstring
      {.emit:[s, "= c_astToStrImpl(", T, ");"].}
      $s
    fun2()

template c_sizeof(T: typedesc): int =
  block:
    # proc needed pending https://github.com/nim-lang/Nim/issues/13943 D20200409T215527
    proc fun2(): int =
      {.emit:[result," = sizeof(", T, ");"].}
    fun2()

macro test(body: untyped): untyped =
  result = newStmtList()
  for T in body:
    result.add quote do:
      let s1 = `T`.sizeof
      let s2 = `T`.c_sizeof
      if s1 != s2:
        echo "$1 $2 => $3 vs $4" % [$astToStr(`T`), c_astToStr(`T`), $s1, $s2]
      # check s1 == s2

test:
  Dirent
  Tflock
  Glob
  Group
  Time
  Utsname
  Ipc_perm
  Stat
  Statvfs
  SigEvent
  # Posix_typed_mem_info # BUG? # error: invalid application of 'sizeof' to an incomplete type 'struct posix_typed_mem_info'