More concept fixes

Fixes #7705, #7703, #7702

tests/concepts/treversable.nim

@@ -0,0 +1,31 @@
+# issue 7705, 7703, 7702
+discard """
+  output: '''
+z
+e
+  '''
+"""
+
+type
+  Reversable*[T] = concept a
+    a[int] is T
+    a.high is int
+    a.len is int
+    a.low is int
+
+proc get[T](s: Reversable[T], n: int): T =
+  s[n]
+
+proc hi[T](s: Reversable[T]): int =
+  s.high
+
+proc lo[T](s: Reversable[T]): int =
+  s.low
+
+iterator reverse*[T](s: Reversable[T]): T =
+  assert hi(s) - lo(s) == len(s) - 1
+  for z in hi(s).countdown(lo(s)):
+    yield s.get(z)
+
+for s in @["e", "z"].reverse:
+  echo s