Add tiny API to make non-allocating tmpl renderers easy. Also update

`examples/tmpl.nim` to use this by default with some side comments for
other ways to use. { Tagging @SirNickolas since seems relevant to
#224 }  String compares even in
a `case` theoretically able to use perfect hashes will, of course,
still be slow compared to char-compares like in `bu/tmpls.nim`, but
folks outside the APL/kdb family of PLs sure love >1 char idents. ;-)
The `fmtUncertain*` family should probably use this new `MacroCallX`,
for example as numbers are a thing of which there can be many.

cligen/strUt.nim

@@ -459,6 +459,12 @@ iterator tmplParse*(fmt: openArray[char], meta='$', ids=alphaNum): MacroCall =
 proc tmplParsed*(fmt: openArray[char], meta='$', ids=alphaNum): seq[MacroCall] =
   ## Pre-process a format string template into a `seq[MacroCall]`.
   for macCall in tmplParse(fmt, meta, ids): result.add macCall
+
+type MacroCallX* = (Slice[int], Slice[int], Slice[int], string) ## Call & string
+proc tmplParses*(fmt: openArray[char], meta='$', ids=alphaNum):seq[MacroCallX] =
+  ## Pre-process a format string template into a `seq[MacroCallX]`.
+  for mc in tmplParse(fmt, meta, ids):
+    result.add (mc[0], mc[1], mc[2], fmt[mc[0]].toString)
                                         #*** FORMATTING UNCERTAIN NUMBERS ***
 const pmUnicode* = "±"                  ## for re-assign/param passing ease
 const pmUnicodeSpaced* = " ± "          ## for re-assign/param passing ease

examples/tmpl.nim

@@ -2,10 +2,11 @@ from std/strutils import toUpperAscii
 import cligen/strUt     # Example/test text template/macro expand/interpolation.
 
 proc render(fmt: openArray[char]): string =
-  for (id, arg, call) in tmplParse(fmt):
+  let fmtParses = fmt.tmplParses       # or `fmt.tmplParsed|iterator tmplParse`
+  for (id, arg, call, idStr) in fmtParses: # or (id, arg, call) with..
     if id.idIsLiteral: result.add fmt[arg]
     else:
-      case fmt[id].toString
+      case idStr                           #..or `fmt[id].toString`
       of "a": result.add "hmm"
       of "bc": result.add "hoo"
       of "uc": (for i in arg: result.add fmt[i].toUpperAscii)