[morley-debugger#13] Preserve nested variable annotations

Description

Problem: According to the reference implementation, the Pair value should preserve variable annotations inside itself. However, Morley's NTPair doesn't preserve these annotations.

Solution: Change NTNotes so that NTPair accepts two variable annotations. This causes many places to require changes as well, such as the parser, packer, unpacker etc. Additionally, the untyped TPair was also changed to accept these two variable annotations.

While resolving this, move one test to its own file to simplify things.

See also: morley-debugger!13 (merged)

Related issue(s)

Pre-condition to resolve morley-debugger#13 (closed)

Resolves morley-debugger#14 (closed)

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