Get rid of 'BurstValueExpressionNode'

[bytorbrynden]

Having all values be contained within a BurstValueExpressionNode, seems unecessary.
The reason I originally created that node, was because I wanted a nice way to keep track of value expression types (i.e. "literal", "expression", etc.), which made sense.

Now that I've used it a bit, I really don't like having it, as it adds an extra layer of ambiguity.
In my eyes, a BurstValueExpressionNode, is no different than a BurstASTNode, which also keeps track of types and node-values.