Rename mget to []

- In sets, tables, strtabs, critbits, xmltree
- This uses the new var parameter overloading
- mget variants still exist, but are deprecated in favor of `[]`
- Includes tests and fixed tests and usages of mget
- The non-var `[]` now throws an exception instead of returning binary 0
  or an empty string

tests/actiontable/tactiontable.nim

@@ -4,24 +4,23 @@ discard """
 
 import tables
 
-proc action1(arg: string) = 
+proc action1(arg: string) =
   echo "action 1 ", arg
 
-proc action2(arg: string) = 
+proc action2(arg: string) =
   echo "action 2 ", arg
 
-proc action3(arg: string) = 
+proc action3(arg: string) =
   echo "action 3 ", arg
 
-proc action4(arg: string) = 
+proc action4(arg: string) =
   echo "action 4 ", arg
 
 var
   actionTable = {
-    "A": action1, 
-    "B": action2, 
-    "C": action3, 
+    "A": action1,
+    "B": action2,
+    "C": action3,
     "D": action4}.toTable
 
 actionTable["C"]("arg")
-