Handle subtype relations for converter parameters (#8248)

Fixes #7098

tests/converter/t7098.nim

@@ -0,0 +1,31 @@
+type
+  Byte* = uint8
+  Bytes* = seq[Byte]
+
+  BytesRange* = object
+    bytes: Bytes
+    ibegin, iend: int
+
+proc initBytesRange*(s: var Bytes, ibegin = 0, iend = -1): BytesRange =
+  let e = if iend < 0: s.len + iend + 1
+          else: iend
+  assert ibegin >= 0 and e <= s.len
+
+  shallow(s)
+  result.bytes = s
+  result.ibegin = ibegin
+  result.iend = e
+
+converter fromSeq*(s: Bytes): BytesRange =
+  var seqCopy = s
+  return initBytesRange(seqCopy)
+
+type
+  Reader* = object
+    data: BytesRange
+    position: int
+
+proc readerFromBytes*(input: BytesRange): Reader =
+  discard
+
+let r = readerFromBytes(@[])