getFileSize and setFileSize procedures for File and AsyncFile

Platform independent procedure to set a file length. Useful when
replacing file content.

tests/async/tasyncfile.nim

@@ -11,9 +11,10 @@ proc main() {.async.} =
   # Simple write/read test.
   block:
     var file = openAsync(fn, fmReadWrite)
-    await file.write("test")
+    await file.write("testing")
     file.setFilePos(0)
     await file.write("foo")
+    file.setFileSize(4)
     file.setFilePos(0)
     let data = await file.readAll()
     doAssert data == "foot"
@@ -24,7 +25,7 @@ proc main() {.async.} =
     var file = openAsync(fn, fmAppend)
     await file.write("\ntest2")
     let errorTest = file.readAll()
-    echo await errorTest
+    yield errorTest
     doAssert errorTest.failed
     file.close()
     file = openAsync(fn, fmRead)