Add windows libffi.dll and remove old code

libffi.nim

@@ -11,36 +11,15 @@
 
 when defined(windows):
   import os
-  # on Windows we don't use a DLL but instead embed libffi directly:
 
-  const thisPath = currentSourcePath.parentDir
-  const flag = "-I" & (thisPath / "common") & " -I" & thisPath
-  {.passc: flag.}
+  const libPath = currentSourcePath.parentDir / "libs"
 
-  {.pragma: mylib, header: r"common/ffi.h".}
-
-  #{.compile: r"common\malloc_closure.c".}
-  {.compile: r"common\raw_api.c".}
-  when defined(vcc):
-    {.compile: r"msvc\ffi.c".}
-    {.compile: r"msvc\prep_cif.c".}
-    {.compile: r"msvc\win32.c".}
-    {.compile: r"msvc\types.c".}
-    when defined(cpu64):
-      {.compile: r"msvc\win64_asm.asm".}
-    else:
-      {.compile: r"msvc\win32_asm.asm".}
+  when defined(cpu64):
+    const arch = "win64"
   else:
-    {.compile: r"gcc\ffi.c".}
-    {.compile: r"gcc\prep_cif.c".}
-    {.compile: r"gcc\types.c".}
-    {.compile: r"gcc\closures.c".}
-    when defined(cpu64):
-      {.compile: r"gcc\ffi64.c".}
-      {.compile: r"gcc\win64_asm.S".}
-    else:
-      {.compile: r"gcc\win32_asm.S".}
+    const arch = "win32"
 
+  {.pragma: mylib, dynlib: libPath / arch / "libffi-7.dll".}
 elif defined(macosx):
   {.pragma: mylib, dynlib: "libffi.dylib".}
 else: