Bump MSRV to 1.78

[popzxc]

Bumps MSRV to 1.79 1.78. Two reasons:

1. 1.70 is too old, and CI is broken. At least 1.74 is required.
2. Output of cargo has changed, and it's better for tests to reflect the current state of affairs.

Changes were introduced in 1.78
Also fixes newly added clippy lints.

[pksunkara]

Output of cargo has changed, and it's better for tests to reflect the current state of affairs.

Which version is that? I would prefer to set either that or 1.74 as MSRV whichever is higher. Setting 1.79 might break a few people's workflows.

[popzxc]

Changes were introduced in 1.78 (PR).

Changed the PR accordingly.