tables.nim: Add named fields in smallest and largest (#14919)

The `smallest` and `largest` procs for `CountTable` returned a tuple
with named fields, but the same procs for `CountTableRef` returned an
anonymous tuple.

This commit makes those `CountTableRef` procs more consistent, and adds
a test.

Fixes: #14918

lib/pure/collections/tables.nim

@@ -2670,14 +2670,14 @@ proc inc*[A](t: CountTableRef[A], key: A, val = 1) =
     doAssert a == newCountTable("aaabbbbbbbbbbb")
   t[].inc(key, val)
 
-proc smallest*[A](t: CountTableRef[A]): (A, int) =
+proc smallest*[A](t: CountTableRef[A]): tuple[key: A, val: int] =
   ## Returns the ``(key, value)`` pair with the smallest ``val``. Efficiency: O(n)
   ##
   ## See also:
   ## * `largest proc<#largest,CountTableRef[A]>`_
   t[].smallest
 
-proc largest*[A](t: CountTableRef[A]): (A, int) =
+proc largest*[A](t: CountTableRef[A]): tuple[key: A, val: int] =
   ## Returns the ``(key, value)`` pair with the largest ``val``. Efficiency: O(n)
   ##
   ## See also:

tests/collections/ttables.nim

@@ -325,6 +325,20 @@ block tablesref:
         else: break
         inc i
 
+  block smallestLargestNamedFieldsTest: # bug #14918
+    const a = [7, 8, 8]
+
+    proc testNamedFields(t: CountTable | CountTableRef) =
+      doAssert t.smallest.key == 7
+      doAssert t.smallest.val == 1
+      doAssert t.largest.key == 8
+      doAssert t.largest.val == 2
+
+    let t1 = toCountTable(a)
+    testNamedFields(t1)
+    let t2 = newCountTable(a)
+    testNamedFields(t2)
+
   block SyntaxTest:
     var x = newTable[int, string]({:})
     discard x