Proto: expose length function in raw-context

Context

This MR corrects the implementation of length for storage and exposes it in Raw_context.

The function length was introduced in !4138 (merged) for Irmin and further in the environment in !4086 (merged). It will be used in a follow up MR !3491 (merged) for improving gas model for Big_map.list_values.

Manually testing the MR

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).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR