Add tests and a Sizable trait for ContextElement.

js/test_cases/helpers_size/README.md

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+Excerpt from [the DustJS tutorial](https://github.com/linkedin/dustjs/wiki/Dust-Tutorial#size_keyxxx___size_helper_Available_in_Dust_V11_release):
+
+    Array - number of elements, [1,2,3,4] has size=4
+    String - length of the string, "abcdef" has size=6
+    Object - Number of properties in the object, {a:4, b:8, c:15, d:16} has size=4
+    Number - Value of the number, 23 has size 23 and 3.14 has size 3.14
+    Undefined, 0, empty string - zero
+    Any other value - length after conversion to string
+

js/test_cases/helpers_size/inputarray.json

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+{
+  "val": [
+    "Alice",
+    "Bob",
+    "Chris"
+  ]
+}

js/test_cases/helpers_size/inputemptystring.json

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+{
+  "val": ""
+}

js/test_cases/helpers_size/inputfalse.json

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+{
+  "val": false
+}

js/test_cases/helpers_size/inputmissing.json

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+{
+  "notval": "Alice"
+}

js/test_cases/helpers_size/inputnumber.json

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+{
+  "val": 7.21
+}

js/test_cases/helpers_size/inputobject.json

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+{
+  "val": {
+    "pet": "cat",
+    "name": "fluffy"
+  }
+}

js/test_cases/helpers_size/inputstring.json

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+{
+  "val": "Alice"
+}

js/test_cases/helpers_size/inputtrue.json

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+{
+  "val": true
+}

js/test_cases/helpers_size/inputzero.json

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+{
+  "val": 0
+}

js/test_cases/helpers_size/main.dust

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+The size of val ({val|js|s}) is {@size key=val /}{~n}
+The size of "{val}" is {@size key="{val}" /}{~n}
+The size with no key is {@size /}{~n}