add alignTable, parseTableCells to align/format a tab(etc) delimited table (#10182)

* add compiler/unittest_light.nim for easy diffing: assertEquals and mismatch
* fixup
* add alignTable, parseTableCells

compiler/unittest_light.nim

@@ -0,0 +1,37 @@
+# note: consider merging tests/assert/testhelper.nim here.
+
+proc mismatch*[T](lhs: T, rhs: T): string =
+  ## Simplified version of `unittest.require` that satisfies a common use case,
+  ## while avoiding pulling too many dependencies. On failure, diagnostic
+  ## information is provided that in particular makes it easy to spot
+  ## whitespace mismatches and where the mismatch is.
+  proc replaceInvisible(s: string): string =
+    for a in s:
+      case a
+      of '\n': result.add "\\n\n"
+      else: result.add a
+
+  proc quoted(s: string): string = result.addQuoted s
+
+  result.add "\n"
+  result.add "lhs:{\n" & replaceInvisible(
+      $lhs) & "}\nrhs:{\n" & replaceInvisible($rhs) & "}\n"
+  when compiles(lhs.len):
+    if lhs.len != rhs.len:
+      result.add "lhs.len: " & $lhs.len & " rhs.len: " & $rhs.len & "\n"
+    when compiles(lhs[0]):
+      var i = 0
+      while i < lhs.len and i < rhs.len:
+        if lhs[i] != rhs[i]: break
+        i.inc
+      result.add "first mismatch index: " & $i & "\n"
+      if i < lhs.len and i < rhs.len:
+        result.add "lhs[i]: {" & quoted($lhs[i]) & "} rhs[i]: {" & quoted(
+            $rhs[i]) & "}"
+      result.add "lhs[0..<i]:{\n" & replaceInvisible($lhs[
+          0..<i]) & "}\nrhs[0..<i]:{\n" & replaceInvisible($rhs[0..<i]) & "}"
+
+proc assertEquals*[T](lhs: T, rhs: T) =
+  when false: # can be useful for debugging to see all that's fed to this.
+    echo "----" & $lhs
+  doAssert lhs==rhs, mismatch(lhs, rhs)