More operator madness.

* Adds an 'in' operator, testing if the lefthand side exists in the righthand side. Evaluates to true/false.
* Overloads the addition operator to allow for more intuitive array concatenation/appending.
* Allow indexing of strings as a shorthand form of String.prototype.charAt()
* Fixed typo in BinaryOperatorExpression.cs
Allow underscores as digit separators in number literals.
Some small tweaks.

* Remove overloaded addition operator for arrays.
* Remove readonly indexer for strings.
* Change 'in' operator to not require a string key on the lefthand side when the righthand side is an object.
* Add toString() and serialize() instance functions to the value prototype.
Merge pull request #6 from Syke94/master

Operator madness & QoL
Merge branch 'master' into experimental