Ensure finish() called on builders before dropped

[ss2165]

Currently finish() implementations just perform local validation checks and then return a handle. If they do more serious things like complete the build (actually mutate the HUGR) we may want a mechanism to ensure it is called before the builder is dropped (indeed, that finish() is the only way to drop the builder). However, this is now wondering in to linear types, so: rust-lang/rfcs#814