perf: buffer IO for Keystore (de)serialization

[Oppen]

What ❔

Use BufReader and BufWriter rather than serializing to and
deserializing from memory, reducing one of the memory usage peaks caused
by temporaries.
For one particular case this avoids a 13GiB allocation.

Why ❔

The CPU prover experiences high memory usage that can bring down some instances.
This is an ongoing effort to reduce sailing and peak memory usage.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zk fmt and zk lint.
• Spellcheck has been run via zk spellcheck.
• Linkcheck has been run via zk linkcheck.

[EmilLuta]

I tried testing the PR, but that's not possible as we're in the middle of 1.5.0 upgrade. Will follow up once the upgrade is done.

[EmilLuta]

P.S. There are a few unused variables and warning lints that can be addressed. I'd appreciate if we do that.

warning: unused imports: `Read`, `self`
 --> vk_setup_data_generator_server_fri/src/keystore.rs:2:10
  |
2 |     fs::{self, File},
  |          ^^^^
3 |     io::{BufReader, BufWriter, Read},
  |                                ^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: variable does not need to be mutable
   --> vk_setup_data_generator_server_fri/src/keystore.rs:271:13
    |
271 |         let mut file = File::open(filepath.clone())
    |             ----^^^^
    |             |
    |             help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

[Oppen]

I tried testing the PR, but that's not possible as we're in the middle of 1.5.0 upgrade. Will follow up once the upgrade is done.

If possible, comment when that's done if you need any update to the code. I check Github notifications so I'll see it.

Re: warnings: of course, I'll fix those soon.

[EmilLuta]

We merged 1.5.0. Let's merge upstream and then we can test it.

[Oppen]

Rebased and fixed the warnings.