[module] Finalize definitions for the end-user

[jyn514]

• This has been discussed in issue cranelift-module should document that finalize_definitions needs to be called before finish #1288
• A short description of what this does, why it is needed; if the
  description becomes long, the matter should probably be discussed in an issue
  first.

This removes the need for end-users of cranelift-module to call module.finalize_definitions before module.finish is called. It is fine for finalize_definitions to remain pub because the implementation calls .drain() on all relevant data, so it will never be finalized twice.

• This PR contains test cases, if meaningful.

N/A, no new features.

• A reviewer from the core maintainer team has been assigned for this PR.
  If you don't know who could review this, please indicate so and/or ping
  bnjbvr. The list of suggested reviewers on the right can help you.

I am not sure who should review this. r? @bnjbvr

[bnjbvr]

It seems fine to me, and from my reading of the issue, to @philipc too, so I'll approve (and let some time for @philipc / @bjorn3 to chime in, if they have a different opinion). Thanks!

[bjorn3]

There is no reason to allow calling finish without calling finalize_definitions.

[philipc]

LGTM

[bnjbvr]

Thanks! Merging then.

[iximeow]

just an appreciation comment: I tripped across finalize_definitions not being called when first working out #902, and found that my use of #902 had become incorrect because cranelift is nicer by default :) thanks!