Check default parameter value type

[CharlesTaylor7]

Part 2 of #112
Default parameter type checking.

[kaleidawave]

Another great PR. Specification test is great and diagnostic great 👍. Up to 202!

Just added two quick commits on top

• Firstly the merge conflict in diagnostics.rs from the first PR.
• Secondly I swapped the ASTNode::get_position call for A::expression_position for getting the position of where to emit the type checking error. This is because the type checking API is meant to be agnostic from a parser implementation (so in the future any other project can reuse the checking APIs without them to being specific to the ezno-parser parser I built). A::expression_position comes from ASTImplementation which works in under any AST definitions that implement it. For the ezno-parser AST definition it does use ASTNode::get_position. That fixes why the CI error on the extras tests as the only place you can refer to ezno-parser in the checker is under the synthesis directory (which mostly connects APIs to the AST definitions). Little bit complex, need to document the ASTImplementation trait and that whole system better!