Add missing type annotations to core CIL classes

[Kemalau]

This PR adds the remaining missing type annotations in the small core CIL helper classes touched by issue #8.

Changes:

• annotate CilMethodBodyFlags.__str__, CilMethodBodyFlags.__repr__, and nested _to_print
• annotate Instruction.__init__ and Instruction.mnemonic
• annotate OpCode.__init__

Validation:

• mypy dncil
• pytest -q

Closes #8

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