refactoring, fixes yet another strictFuncs regression (#15446)

tests/effects/tstrict_funcs.nim

@@ -0,0 +1,17 @@
+discard """
+  cmd: "nim c --experimental:strictFuncs $file"
+"""
+
+import tables, streams, nre, parsecsv
+
+type
+  Contig2Reads = TableRef[string, seq[string]]
+
+proc get_Contig2Reads(sin: Stream, fn: string, contig2len: TableRef[string, int]): Contig2Reads =
+  result = newTable[string, seq[string]]()
+  var parser: CsvParser
+  open(parser, sin, filename = fn, separator = ' ', skipInitialSpace = true)
+  while readRow(parser, 2):
+    if contig2len.haskey(parser.row[1]):
+      mgetOrPut(result, parser.row[1], @[]).add(parser.row[0])
+