ortho: track number of trapezoids along with its array

ortho: track number of trapezoids along with its array

This is an alternative to passing around `ntraps` or relying on the `TRSIZE`
computation. It is a step towards fixing an issue where the allocated number of
trapezoids is exceeded.

The diff looks large but most of this change is mechanical, migrating `tr` uses
to be aware of the new structure.

This introduces some new warnings, but the intention is to remove these in
upcoming commits as we switch to on-demand allocation.

Gitlab: #56