[Closes #106] Hold Arc<SpinLock<OpenedFileTable>> instead of just SpinLock<>

[michalfita]

Description

Part of preparation to support the whole clone3() syscall. This change allows threads to keep access to the shared table of opened files (in #88 and #115), but for now it retains just cloning the table into new Arc<SpinLock<>> on fork().

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• Fixed all cargo clippy warnings.
• Applied rustfmt.

[nuta]

Merged. Thanks!