implement box avoidance

[mgravell]

fix #2638

1. adds internal BoxedPrimitives helper that holds boxed instances for common primitive values and has helper APIs
2. use BoxedPrimitives in preference to vanilla boxing in JValue and JsonTextReader
3. avoid new JValue(object) in key paths

Disclosure: if someone uses Unsafe methods to mutate the inside of one of these boxes: bad things. But that is self-inflicted.

[mgravell]

@JamesNK nits resolved

[JamesNK]

Does this have any impact on comparisons when the value is still boxed? I wonder about people doing something dumb like:

if (JValue1.Value == JValue2.Value)
{
}

Before it would be false but now it is true (if values are -1 to 8 and share a box)

[mgravell]

@JamesNK yes, technically the change is observable; however, the code as shown is already incorrect for most definitions of "correct"/"incorrect"; when dealing with object, what they almost certainly mean is something like:

if (Equals(JValue1.Value, JValue2.Value))
{
}

or

if (JValue1.Value.Equals(JValue2.Value))
{
}

The == usage is simply ... wrong. It will already behave very inconsistently. For example, if the values are strings, it would currently report true for two empty strings but only two empty strings. The following passes against the old or new code:

        [Test]
        public void EmptyStringsAreSame()
        {
            var obj = (JObject)JToken.Parse(@"{""x"": """", ""y"": """"}");
            var x = ((JValue)obj["x"]).Value;
            var y = ((JValue)obj["y"]).Value;
            Assert.AreSame(x, y);
            Assert.AreEqual(x, y);
            Assert.IsTrue(x == y);
        }

        [Test]
        public void EqualStringsAreNotSame()
        {
            var obj = (JObject)JToken.Parse(@"{""x"": ""a"", ""y"": ""a""}");
            var x = ((JValue)obj["x"]).Value;
            var y = ((JValue)obj["y"]).Value;
            Assert.AreNotSame(x, y);
            Assert.AreEqual(x, y);
            Assert.IsFalse(x == y);
        }

I'm not sure we should be too concerned about obviously-broken code.

[JamesNK]

Thanks @mgravell