[cleanup] Make Function code nicer

[georgemitenkov]

Description

While adding function values, ended up cleaning up function code which was not in a great state. Changes:

1. Stop using pub(crate) for Function fields,
2. Group fields as type signature, internals (native or bytecode).

How Has This Been Tested?

Key Areas to Review

Type of Change

• Refactoring

Which Components or Systems Does This Change Impact?

• Move Virtual Machine

Checklist

• I have read and followed the CONTRIBUTING doc
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I identified and added all stakeholders and component owners affected by this change as reviewers
• I tested both happy and unhappy path of the functionality
• I have made corresponding changes to the documentation

[trunk-io]

⏱️ 11m total CI duration on this PR

Job	Cumulative Duration	Recent Runs
rust-move-tests	7m	🟥
rust-cargo-deny	2m	🟩
check-dynamic-deps	36s	🟩
general-lints	27s	🟩
semgrep/ci	22s	🟩
file_change_determinator	10s	🟩
permission-check	4s	🟩
permission-check	2s	🟩
check-branch-prefix	1s	🟩

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[georgemitenkov]

• [cleanup] Make Function code nicer #15389 👈 (View in Graphite)
• main

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