Add support for permissive enums

[dmlary]

Problem Statement

When reading structures from memory, it's possible that some enum fields have unknown values. As a result, the CType will raise an exception instead of unpacking the rest of the struct. CTypes::Enum#permissive returns a duplicate Enum type that does not raise an exception when unknown values are encountered.

Types returned by ElfUtils currently are all strict types, raising exceptions for data mismatches.

To make it easier to unpack real-world data, we should add a mechanism to use permissive types instead of strict types.

Proposed Solution

todo