Allow compilation with no slasher backend (#3888)

## Proposed Changes

Allowing compiling without MDBX by running:

```bash
CARGO_INSTALL_EXTRA_FLAGS="--no-default-features" make
```

The reasons to do this are several:

- Save compilation time if the slasher won't be used
- Work around compilation errors in slasher backend dependencies (our pinned version of MDBX is currently not compiling on FreeBSD with certain compiler versions).

## Additional Info

When I opened this PR we were using resolver v1 which [doesn't disable default features in dependencies](https://doc.rust-lang.org/cargo/reference/features.html#resolver-version-2-command-line-flags), and `mdbx` is default for the `slasher` crate. Even after the resolver got changed to v2 in #3697 compiling with `--no-default-features` _still_ wasn't turning off the slasher crate's default features, so I added `default-features = false` in all the places we depend on it.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

book/src/installation-source.md

@@ -133,6 +133,15 @@ Commonly used features include:
 * `slasher-lmdb`: support for the LMDB slasher backend.
 * `jemalloc`: use [`jemalloc`][jemalloc] to allocate memory. Enabled by default on Linux and macOS.
   Not supported on Windows.
+* `spec-minimal`: support for the minimal preset (useful for testing).
+
+Default features (e.g. `slasher-mdbx`) may be opted out of using the `--no-default-features`
+argument for `cargo`, which can plumbed in via the `CARGO_INSTALL_EXTRA_FLAGS` environment variable.
+E.g.
+
+```
+CARGO_INSTALL_EXTRA_FLAGS="--no-default-features" make
+```
 
 [jemalloc]: https://jemalloc.net/