Restrict Decl Nesting.

[csyonghe]

The parser should check for illegal nesting of decls, such as a generic nested inside a generic, or a import nesting inside a function.

The solution is to declare a table of allowed decls for each decl, and perform that check systematically, e.g.

Dictionary<ASTNodeType, HashSet<ASTNodeType>> kAllowedChildNodeTypes;

For now, we can allow a fallback case that if a decl type does not have an entry in kAllowedChildNodeTypes, then it is allowed anywhere.