coming from https://github.com/denki/listings-rust.

## changelog
- _semi\_avid_, _kzg_ and _aplonk_ examples have been added
- the `fs` module has been hidden behind an `fs` feature
- the `conversions` module has been properly hidden behind the `test` config feature
- the documentation has been completed
- some error messages have been improved

> **Note**
>
> the documentation of aPlonK has been left as-is for now

`algebra`, `field` and `linalg` were doing extremely similar things before...

this MR merges them into a single module `algebra`
- old `algebra` and `field` are at the root of the new `algebra`
- old `linalg` is now `algebra::linalg`

all references to these have been fixed in the rest of the codebase and the features have been tuned to work fine.

in !162 and !163, i did only run `cargo ... --all-features` without checking the individual features...

this MR adds some `cargo check --features ...` to the `Makefile` and makes sure the imports make sense when compiling a single feature.

> ❗ **Important**
>
> the other notable change here is that `kzg::commit` has been moved to `zk::ark_commit` and is re-exported from `kzg` as `kzg::commit`.