colors: Introduce resetStyle enum & use templates

1. Introduce TerminalCmd enum and resetStyle that can be issued to issue
   resetAttributes within styledEcho arguments.

2. Use templates to resolve styledEcho arguments for performance reasons.

3. Try to avoid calling trailing write "\n" and reset attributes where possible.

lib/pure/terminal.nim

@@ -406,22 +406,43 @@ proc isatty*(f: File): bool =
 
   result = isatty(getFileHandle(f)) != 0'i32
 
-proc styledEchoProcessArg(s: string) = write stdout, s
-proc styledEchoProcessArg(style: Style) = setStyle({style})
-proc styledEchoProcessArg(style: set[Style]) = setStyle style
-proc styledEchoProcessArg(color: ForegroundColor) = setForegroundColor color
-proc styledEchoProcessArg(color: BackgroundColor) = setBackgroundColor color
+type
+  TerminalCmd* = enum  ## commands that can be expressed as arguments
+    resetStyle         ## reset attributes
+
+template styledEchoProcessArg(s: string) = write stdout, s
+template styledEchoProcessArg(style: Style) = setStyle({style})
+template styledEchoProcessArg(style: set[Style]) = setStyle style
+template styledEchoProcessArg(color: ForegroundColor) = setForegroundColor color
+template styledEchoProcessArg(color: BackgroundColor) = setBackgroundColor color
+template styledEchoProcessArg(cmd: TerminalCmd) =
+  when cmd == resetStyle:
+    resetAttributes()
 
 macro styledEcho*(m: varargs[expr]): stmt =
   ## to be documented.
   let m = callsite()
+  var reset = false
   result = newNimNode(nnkStmtList)
 
   for i in countup(1, m.len - 1):
-    result.add(newCall(bindSym"styledEchoProcessArg", m[i]))
+    let item = m[i]
+    case item.kind
+    of nnkStrLit..nnkTripleStrLit:
+      if i == m.len - 1:
+        # optimize if string literal is last, just call writeln
+        result.add(newCall(bindSym"writeln", bindSym"stdout", item))
+        if reset: result.add(newCall(bindSym"resetAttributes"))
+        return
+      else:
+        # if it is string literal just call write, do not enable reset
+        result.add(newCall(bindSym"write", bindSym"stdout", item))
+    else:
+      result.add(newCall(bindSym"styledEchoProcessArg", item))
+      reset = true
 
   result.add(newCall(bindSym"write", bindSym"stdout", newStrLitNode("\n")))
-  result.add(newCall(bindSym"resetAttributes"))
+  if reset: result.add(newCall(bindSym"resetAttributes"))
 
 when defined(nimdoc):
   proc getch*(): char =