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

tests/errmsgs/t9768.nim

@@ -1,6 +1,6 @@
 discard """
   errmsg: "unhandled exception:"
-  file: "system.nim"
+  file: "system/fatal.nim"
   nimout: '''
 stack trace: (most recent call last)
 t9768.nim(28, 33)        main

tests/manyloc/standalone/panicoverride.nim

@@ -2,10 +2,6 @@
 proc printf(frmt: cstring) {.varargs, importc, header: "<stdio.h>", cdecl.}
 proc exit(code: int) {.importc, header: "<stdlib.h>", cdecl.}
 
-proc nimToCStringConv(s: NimString): cstring {.compilerProc, inline.} =
-  if s == nil or s.len == 0: result = cstring""
-  else: result = cstring(addr s.data)
-
 {.push stack_trace: off, profiler:off.}
 
 proc rawoutput(s: string) =