Support elsif when stringifying If nodes

[HertzDevil]

When an If node's else-clause is itself a non-ternary If node, combines the two nodes and emits an elsif, instead of an else plus an indented if. This is done even when the original nodes did not use an elsif, and applies to arbitrarily long If chains.

[straight-shoota]

I'm not sure if this is okay... IMO stringification should print a representation of the AST node, not implicitly transform it.

If we want such a transformation, perhaps it should better be explicit.

[HertzDevil]

There is no AST transformation per se, because there is simply no such thing as an Elsif node right now. The two representations are syntactically equivalent and this only amounts to picking a different canonical string representation.