unconditionally pass through std/os and deprecate shim/os (#199)

stew/shims/os.nim

@@ -1,18 +1,3 @@
+{.deprecated: "use std/os".}
 import std/os
 export os
-
-when defined(windows):
-  import winlean
-else:
-  import posix
-
-when not declared(getCurrentProcessId):
-  proc getCurrentProcessId*(): int =
-    ## return current process ID. See also ``osproc.processID(p: Process)``.
-    when defined(windows):
-      proc GetCurrentProcessId(): DWORD {.stdcall, dynlib: "kernel32",
-                                        importc: "GetCurrentProcessId".}
-      GetCurrentProcessId().int
-    else:
-      getpid()
-