feat(typeck): extending variable declaration rules

[Jds-23]

#657

• crates/sema/src/ty/mod.rs

  • Error message for public interface parameter/return types updated to reject types that "contain non-public function pointers" (adds check for Ty::has_internal_function).

• crates/sema/src/ty/ty.rs

  • Added Ty::has_internal_function() to query a new flag.
  • can_be_exported() now disallows types that contain non-public (internal/private) function pointers.
  • Added a new TyFlags::HAS_INTERNAL_FN bit.
  • Extended TyFlags logic to handle TyKind::FnPtr:
    • If the fn pointer's visibility is non-public, set HAS_INTERNAL_FN.
    • Propagate flags from the fn pointer's parameter and return types (calls add_tys on parameters and returns).

• crates/sema/src/typeck/checker.rs

  • Added helper is_compile_time_constant() using ConstantEvaluator.
  • check_assign() now enforces rules around assigning types that contain mappings:
    • Disallows assigning storage-containing mapping types unless the lvalue is a local/return variable.
    • Emits an error message for invalid assignments.
  • Added is_local_or_return_variable() to detect when an expr refers to a local or return variable.
  • Variable declaration handling (when visiting variables):
    • Constants must be initialized and initializers must be compile-time constant (emit errors otherwise).
    • Immutable variables must be value types (emit error otherwise).
    • State variables containing mappings cannot have initializers (emit error).
    • Non-state variables in calldata/memory that contain mappings are rejected (emit error, instructing the type is only valid in storage).
    • Preserves prior expect_ty check for initializers when present.