MIR: Add PACK instruction

Context

Depends on !10691 (merged)

Adds PACK that works on values.

As discussed:

• I use the existing function for untyping value, as we don't strictly need pure optimized representation, at least for now.
• I do not add full coverage for PACK, we have TZTs for this.

As for gas, there are 4 stages in interpreting PACK:

• Facing PACK instruction is itself free. Nevertheless, I put that to gas-related consts so that it can be overridden when we become parametrized by the gas model.
• Octez implementation needs to strip locations from the value, we don't.
• Value untyper cost - I do not handle it here, but separately I started some work for the future MR.
• Serialization cost - added.

Manually testing the MR

cargo test

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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