Make #is_a? in macros respect the AST node hierarchy

[HertzDevil]

Currently, is_a? in macros compares the class names directly:

{{ [1, 2, 3].is_a?(ASTNode) }} # => false

# the above is akin to the following in normal code:
[1, 2, 3].class.name == Object.class.name # => false

Since Crystal::ASTNode is an abstract class, it follows that is_a?(ASTNode) always fails, even though every AST node obviously inherits from that class, both in the macro language and internally. This PR makes the internal AST node hierarchy available here, so is_a?(ASTNode) becomes true.

More useful is the ability to match abstract AST node types that aren't ASTNode:

macro foo(x)
  {% puts x.exp if x.is_a?(UnaryExpression) %}
end

foo !true         # => true
foo sizeof(Int32) # => Int32
foo *[1, 2, 3]    # => [1, 2, 3]
foo 4 + 5
foo /bar/

[beta-ziliani]

Thanks once again 🙏