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/tmget.nim

@@ -0,0 +1,141 @@
+discard """
+  output: '''Can't access 6
+10
+11
+Can't access 6
+10
+11
+Can't access 6
+10
+11
+Can't access 6
+10
+11
+Can't access 6
+10
+11
+Can't access 6
+10
+11
+Can't access 6
+5
+Can't access 6
+10
+11
+Can't access 6
+10
+11'''
+"""
+
+import tables
+
+block:
+  var x = initTable[int, int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+block:
+  var x = newTable[int, int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+block:
+  var x = initOrderedTable[int, int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+block:
+  var x = newOrderedTable[int, int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+block:
+  var x = initCountTable[int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+block:
+  var x = newCountTable[int]()
+  x[5] = 10
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+  x[5] += 1
+  var c = x[5]
+  echo c
+
+import sets
+
+block:
+  var x = initSet[int]()
+  x.incl 5
+  try:
+    echo x[6]
+  except KeyError:
+    echo "Can't access 6"
+  echo x[5]
+
+import critbits
+
+block:
+  var x: CritBitTree[int]
+  x["5"] = 10
+  try:
+    echo x["6"]
+  except KeyError:
+    echo "Can't access 6"
+  echo x["5"]
+  x["5"] += 1
+  var c = x["5"]
+  echo c
+
+import strtabs
+
+block:
+  var x = newStringTable()
+  x["5"] = "10"
+  try:
+    echo x["6"]
+  except KeyError:
+    echo "Can't access 6"
+  echo x["5"]
+  x["5"][1] = '1'
+  var c = x["5"]
+  echo c