lib_shell_services: Change raw_context Dir constructor to a Map

Context

Closes #1148 (closed)

Change the type of raw_context.Dir to a raw_context TzString.Map.t (instead of a list of pairs). Doing so puts an invariant in the type system.

Manually testing the MR

CI passes

Checklist

• N/A Document the interface of any function added or modified (see the coding guidelines)
• N/A Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
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