Small farmer refactoring

[nazar-pc]

First commit fixes annoying thing introduced in #1778 where specified farm path must exist before use, now it will be created automatically, but only one level deep to prevent surprising behavior.

Second commit is changes extracted from checksumming branch that tries to improve readability, remove some boilerplate and hide some internals of subspace-farmer-components that do not need to be exposed publicly.

Ignore whitespace changes, then the diff is quite small.

Code contributor checklist:

• I have read, understood and followed contributing guide

[shamil-gadelshin]

Don't you think pub(crate) for the struct only is enough? Is it somewhere in the rust guidelines?

[nazar-pc]

Fields and methods were also made private, what do you mean by struct only?

I was just hiding data structures that are internal to subspace-farmer-components and not exposed in public API. This helps ensuring that when in later PR I'm adding checksums, all users of these data structures are doing checksums the same way consistently.

[shamil-gadelshin]

I meant that by making the struct private you prevent leaking the type outside of the crate and all methods and fields are unaccessible respectively. pub methods and fields are less noisy than pub(crate). But maybe it's considered malpractice by the Rust guideline with a good reason.

[nazar-pc]

I mean if you need to make it visible publicly, you use pub, if only in parent module you use pub(super) and if only inside of the crate then pub(crate). Using pub when it is not exposed publicly is misleading and also results in data structures not reported as unused after refactoring. So everything that can be private should be private and as private as possible.