This PR fixes some clippy lints about lifetime annotations which occur
when running clippy like it is run in CI using the nightly toolchain.

More precisely, when running:
```bash
cargo clippy --all-features --lib --bins --examples --tests -- -D warnings
```
or alternatively `just clippy` when #845 is merged, one gets multiple
warnings (Errors because of `-D warnings`) of the following sort:
```bash
error: lifetime flowing from input to output with different syntax can be confusing
   --> src/adj.rs:314:25
    |
314 |     pub fn edge_indices(&self) -> EdgeIndices<E, Ix> {
    |                         ^^^^^     ------------------ the lifetime gets resolved as '_
    |                         |
    |                         this lifetime flows to the output
    |
    = note: -D mismatched-lifetime-syntaxes implied by -D warnings
    = help: to override -D warnings add #[allow(mismatched_lifetime_syntaxes)]
help: one option is to remove the lifetime for references and use the anonymous lifetime for paths
    |
314 |     pub fn edge_indices(&self) -> EdgeIndices<'_, E, Ix> {
    |                                               +++
```

This PR fixes these lints by exactly adding an anonymous lifetime to the
return types and thus making the lifetime flow more explicit.
Furthermore, a tiny change in the order of `std` and `no-std` generic
parameters in `matrix_graph.rs` is made to make RustRover lints happy.

…ph#845)

This addresses the problem with CONTRIBUTING.md found in petgraph#844. 

Additionally, further details about CI actions which might be
interesting to contributors were added such as miri, as well as running
tests on different toolchains / rust versions.

Resolves petgraph#844

This PR fixes some clippy lints about lifetime annotations which occur
when running clippy like it is run in CI using the nightly toolchain.

More precisely, when running:
```bash
cargo clippy --all-features --lib --bins --examples --tests -- -D warnings
```
or alternatively `just clippy` when petgraph#845 is merged, one gets multiple
warnings (Errors because of `-D warnings`) of the following sort:
```bash
error: lifetime flowing from input to output with different syntax can be confusing
   --> src/adj.rs:314:25
    |
314 |     pub fn edge_indices(&self) -> EdgeIndices<E, Ix> {
    |                         ^^^^^     ------------------ the lifetime gets resolved as '_
    |                         |
    |                         this lifetime flows to the output
    |
    = note: -D mismatched-lifetime-syntaxes implied by -D warnings
    = help: to override -D warnings add #[allow(mismatched_lifetime_syntaxes)]
help: one option is to remove the lifetime for references and use the anonymous lifetime for paths
    |
314 |     pub fn edge_indices(&self) -> EdgeIndices<'_, E, Ix> {
    |                                               +++
```

This PR fixes these lints by exactly adding an anonymous lifetime to the
return types and thus making the lifetime flow more explicit.
Furthermore, a tiny change in the order of `std` and `no-std` generic
parameters in `matrix_graph.rs` is made to make RustRover lints happy.

…ph#845)

This addresses the problem with CONTRIBUTING.md found in petgraph#844. 

Additionally, further details about CI actions which might be
interesting to contributors were added such as miri, as well as running
tests on different toolchains / rust versions.

Resolves petgraph#844