Make Crystal::Select::When inherit from Crystal::ASTNode

[HertzDevil]

This doesn't affect the macro API, because no AST node methods are available on Select yet. It reserves the macro type name SelectWhen though (note the lack of a namespace).

[straight-shoota]

I find the disparity between Select::When and SelectWhen quite confusing. And then there's also When (for case ... when), which makes me wonder if there's even a need for distinct data types for when in different contexts 🤔

Perhaps an easy solution could be to move Select::When to When?

[HertzDevil]

That way you'd always have to remember to write conds.first whenever dealing with select expressions, which might be a tradeoff. But maybe it isn't such a big issue since the compiler has seen wilder reuse of AST node types (e.g. Arg as enum member)

[straight-shoota]

I suppose a benefit of having distinct types is a smaller memory footprint. There's no need to allocate an array for conds if it can only be one item.