Simplify ApplyTcpOptionsError

[pronebird]

ApplyTcpOptionsErrorInternal is superficial and can be replaced with a Box<>, especially because std::error::Error::source() returns Option<&dyn std::error::Error>. Identical approach is already used in std::io::Error, so familiarity bonus point here.

Improvements in this PR

• Remove variant mirroring between ApplyTcpOptionsErrorKind and ApplyTcpOptionsErrorInternal.
• RemoveApplyTcpOptionsErrorInternal.

Further thoughts

• ApplyTcpOptionsError::new(.., ..) is a bit verbose, mostly because of how types are called. I thought of droppingApplyTcpOptions from error-related type names to avoid repeating the module name in it. However I didn't quite agree with myself that this is an improvement.

• tcp_options::* are re-exported in lib.rs, hard to say why this module gets a special treatment, but then I don't know much about the use cases here. Maybe the consumer of this library needs to set socket options frequently.

• There is a performance impact when using Box and dynamic dispatch. But this is negligible since error code paths should not be traversed frequently.

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