Scalar type checks for AST classes

[stevan]

Currently the auto-generated AST classes will type check most arguments correctly, however it does not actually support sensible checks for the Scalar types (String, Boolean, Int, etc). Currently these are only checked to be defined values, but we can do better.

Most of this can be done within the bin/util/generate-ast-classes script.

Here are the locations that deal with scalars (non-blessed/non-ref) in the script:

• https://github.com/stevan/p5-Graph-QL/blob/master/bin/util/generate-ast-classes#L179
• https://github.com/stevan/p5-Graph-QL/blob/master/bin/util/generate-ast-classes#L193
• https://github.com/stevan/p5-Graph-QL/blob/master/bin/util/generate-ast-classes#L206

The way to do this is to write new assertion checkers and put them into the Graph::QL::Util::Assertions module, and then have the script use them. We need to move towards centralizing the type checking logic anyway.