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/stdlib/tmitems.nim

@@ -132,5 +132,5 @@ block:
   </Students>""")
   for x in d.mitems:
     x = <>Student(Name=x.attrs["Name"] & "foo")
-  d.mget(1).attrs["Name"] = "bar"
+  d[1].attrs["Name"] = "bar"
   echo d