move assertions and iterators out of system.nim (#10597)

* move assertions and iterators out of system.nim
* limit nimsuggest tests to the first 3 suggestions

tests/assert/tfailedassert_stacktrace.nim

@@ -5,9 +5,9 @@ discard """
 const expected = """
 tfailedassert_stacktrace.nim(34) tfailedassert_stacktrace
 tfailedassert_stacktrace.nim(33) foo
-system.nim(*)         failedAssertImpl
-system.nim(*)         raiseAssert
-system.nim(*)         sysFatal"""
+assertions.nim(*)       failedAssertImpl
+assertions.nim(*)        raiseAssert
+fatal.nim(*)            sysFatal"""
 
 proc tmatch(x, p: string): bool =
   var i = 0