Revert export of times.CTime; add std/time_t instead. (#10319)

* Revert export of times.CTime
* Add std/time_t

lib/std/time_t.nim

@@ -0,0 +1,23 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2019 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+when defined(nimdoc):
+  type
+    impl = distinct int64
+    Time* = impl ## \
+      ## Wrapper for ``time_t``. On posix, this is an alias to ``posix.Time``.
+elif defined(windows):
+  when defined(i386) and defined(gcc):
+    type Time* {.importc: "time_t", header: "<time.h>".} = distinct int32
+  else:
+    # newest version of Visual C++ defines time_t to be of 64 bits
+    type Time* {.importc: "time_t", header: "<time.h>".} = distinct int64
+elif defined(posix):
+  import posix
+  export posix.Time