ngx::core::Pool's methods are &self, not &mut self

[pchickey]

The interior pointer to ngx_pool_t provides the mutability we need here - there is no requirement for the use of the ngx_pool_t to be unique, so restricting the methods to &mut self just adds additional difficulty without any safety benefit.

Closes #192

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• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• [n/a] I have added tests (when possible) that prove my fix is effective or that my feature works
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• I have updated necessary documentation
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[bavshin-f5]

Makes sense, thanks!
Just reminding that we use conventional commits in this repo. I'd appreciate if you reword the commit message like refactor: take shared ref to self in ngx::core::Pool methods.

The interior ngx_pool_t provides the mutability we need here

interior pointer to ngx_pool_t?

[pchickey]

@bavshin-f5 Both fixed, thanks for the reminder.