100 hours!

Evil hack part 4: Death of evil hack

So I was using a really terrible hack to implement stuff like
FunctionInlining, Safe dot and the elvis expression, the hack is
basically (ab)using the when expression to do stuff like run code inside
expressions and use local variables inside expressions.

So to stop this from becoming a hack, I added it directly to the AST as
StatementExpression(not to be confused with ExpressionStatement), which
can be used to contain statements inside expressions.

All of my evil hacks have been replaced by StatementExpression.

I also added some small optimizations for StatementExpressions!