checker: clean up ensure_type_exists()

[yuyi98]

This PR clean up ensure_type_exists().

[spytheman]

I am not sure that changing ? to false is simpler in all aspects.
In terms of implementation, a boolean is simpler than an option.
However, with that change, the call sites now look like this:

	- c.ensure_type_exists(arg.typ, arg.type_pos) or { return }
	+ if !c.ensure_type_exists(arg.typ, arg.type_pos) {
	+	return
	+ }

[yuyi98]

Minimize the use of option for frequently invoked functions for the performance.

[yuyi98]

There is a problem with using return instead of return none in the original ensure_type_exists(...) ?, so or { return } doesn't work correctly.