Add null violation checks for std::unique_ptr, std::shared_ptr, std::optional and std::expected to prevent UB

[bluetarpmedia]

assert_not_null now verifies std::unique_ptr, std::shared_ptr, std::optional and std::expected in addition to the original null pointer check.

These states are considered to be "null" and will result in a violation error if dereferenced or accessed:

• std::unique_ptr that owns nothing
• std::shared_ptr with no managed object
• std::optional with no value
• std::expected containing an unexpected value

Null violation errors will be reported in the following cases:

test: () = {
    ex: std::expected<int, bool> = std::unexpected(false);
    op: std::optional<int> = std::nullopt;

    up:= unique.new<int>(1);
    sp:= shared.new<int>(2);
    up.reset();
    sp.reset();

    v1 := ex*;   // Null violation error
    v2 := op*;   // Null violation error
    v3 := up*;   // Null violation error
    v4 := sp*;   // Null violation error
}

Add new regression tests:

• pure2-assert-expected-not-null
• pure2-assert-optional-not-null
• pure2-assert-shared-ptr-not-null
• pure2-assert-unique-ptr-not-null

[JohelEGP]

This seems good enough.

It seems like a more general solution would use x is T when CPP2_TYPEOF(x) wraps a T.
But we just can't guarantee that the precondition of x* is the same as x is T.

[hsutter]

Thanks!

[JohelEGP]

The test output isn't platform-independent.
See #556:

• Update tests that finish through std::terminate.
  • Add std::set_terminate(std::abort); to avoid platform-dependent output on termination.