adds an option to interleave the generated code with snippets from the original source

Lines from the original source are outputted as comments next to line directives.
Hopefully, this will make debugging codegen problems easier.

Other changes:
The frame setup code now uses a single-line C macro. My motivation was to reduce
the noise in the generated output and make it easier to step over the boiler-plate
code, but counter-intuitively this also improved the overall compilation speed a
little bit so I applied the same treatment to line tracking too (this reduces the size of
the generated files and the explanation is that probably the I/O overhead dominates
the macro expansion costs).

compiler/ropes.nim

@@ -56,7 +56,7 @@
 #  To cache them they are inserted in a `cache` array.
 
 import 
-  msgs, strutils, platform, hashes, crc, options
+  strutils, platform, hashes, crc, options
 
 type
   TFormatStr* = string # later we may change it to CString for better
@@ -72,6 +72,11 @@ type
   
   TRopeSeq* = seq[PRope]
 
+  TRopesError* = enum
+    rCannotOpenFile
+    rInvalidFormatStr
+    rTokenTooLong
+
 proc con*(a, b: PRope): PRope
 proc con*(a: PRope, b: string): PRope
 proc con*(a: string, b: PRope): PRope
@@ -92,6 +97,9 @@ proc RopeInvariant*(r: PRope): bool
   # exported for debugging
 # implementation
 
+var ErrorHandler*: proc(err: TRopesError, msg: string, useWarning = false)
+  # avoid dependency on msgs.nim
+  
 proc ropeLen(a: PRope): int = 
   if a == nil: result = 0
   else: result = a.length
@@ -225,8 +233,7 @@ proc WriteRope*(head: PRope, filename: string, useWarning = false) =
     if head != nil: WriteRope(f, head)
     close(f)
   else:
-    rawMessage(if useWarning: warnCannotOpenFile else: errCannotOpenFile,
-               filename)
+    ErrorHandler(rCannotOpenFile, filename, useWarning)
 
 var
   rnl* = tnl.newRope
@@ -255,8 +262,8 @@ proc ropef(frmt: TFormatStr, args: varargs[PRope]): PRope =
           inc(i)
           if (i > length + 0 - 1) or not (frmt[i] in {'0'..'9'}): break 
         num = j
-        if j > high(args) + 1: 
-          internalError("ropes: invalid format string $" & $(j))
+        if j > high(args) + 1:
+          ErrorHandler(rInvalidFormatStr, $(j))
         else:
           app(result, args[j - 1])
       of 'n':
@@ -265,7 +272,8 @@ proc ropef(frmt: TFormatStr, args: varargs[PRope]): PRope =
       of 'N':
         app(result, rnl)
         inc(i)
-      else: InternalError("ropes: invalid format string $" & frmt[i])
+      else:
+        ErrorHandler(rInvalidFormatStr, $(frmt[i]))
     var start = i
     while i < length:
       if frmt[i] != '$': inc(i)
@@ -290,8 +298,8 @@ const
 
 proc auxRopeEqualsFile(r: PRope, bin: var tfile, buf: Pointer): bool = 
   if r.data != nil:
-    if r.length > bufSize: 
-      internalError("ropes: token too long")
+    if r.length > bufSize:
+      ErrorHandler(rTokenTooLong, r.data)
       return
     var readBytes = readBuffer(bin, buf, r.length)
     result = readBytes == r.length and