@ASTTest AST transformation

When we develop AST transformations, it is often difficult to check that a particular transformation performs what is necessary. For example, we would like, in type checking, to verify that after the type checker has performed its work, a particular AST node contains metadata about the inferred type. Checking this currently requires compilation trickery which is not user-friendly. We propose to add an AST transformation which allows embedding tests. For example, we could test the @Field AST transformation this way:

import groovy.transform.*

@ASTTest(value = {
    def owner = node.declaringClass
    assert owner.fields.any { it.name == 'x' }
}
@Field int x