Nesting of Type Contexts, Type Hints, and Improved Array Typing

This MR introduces a few extensions to the typifier, allowing it to infer types for a wider range of more complex expressions. In particular, the typing of array literals and array declaration is improved significantly.

• Allow nested type contexts to be deferred by linking them to their parent contexts via inference hooks
• Introduce a system of type hints and their propagation through inference hooks in order to
  • resolve type contexts from incomplete type information
  • propagate fallback-to-default behaviour to nested contexts
  • (possible future applications)
• Use the above to refactor and extend the type inference of array literals and subscripts:
  • Array literals now fall back to the default type if no array type is known
  • Inline array literals can now inherit their type from the enclosing expression's type context
• Apply the default numeric data type to arguments of PsCast and relationals if the argument type could not be inferred

Closes #99 (closed).