Ensure Crystal::Visitor#visit returns Bool

[HertzDevil]

A lot of overloads implicitly return nil or the last expression of the def body, from Arrays and IOs to Crystal::Types and LLVM::Values, meaning more unnecessary work for the compiler, since Crystal::Visitor#visit is only called inside Crystal::ASTNode#accept and only the truthiness matters.

Most of the missing returns are for AST nodes that already have no children, so this is unlikely to affect compiler performance significantly.