added system.getStackTrace; docgen refactoring (incomplete)

lib/system.nim

@@ -1897,6 +1897,9 @@ when not defined(EcmaScript) and not defined(NimrodVM):
   proc writeStackTrace*()
     ## writes the current stack trace to ``stderr``. This is only works
     ## for debug builds.
+
+  proc getStackTrace*(): string
+    ## gets the current stack trace. This is only works for debug builds.
     
   {.push stack_trace: off.}
   when hostCPU == "avr":

lib/system/excpt.nim

@@ -246,6 +246,13 @@ proc WriteStackTrace() =
   else:
     writeToStdErr("No stack traceback available\n")
 
+proc getStackTrace(): string =
+  when hasSomeStackTrace:
+    result = ""
+    rawWriteStackTrace(result)
+  else:
+    result = "No stack traceback available\n"
+
 when defined(endb):
   var
     dbgAborting: bool # whether the debugger wants to abort