Optionally allow expect and unwrap in tests

[smoelius]

This addresses #1015, except it makes the new behavior optional.

The reason for the msrv-related changes is as follows.

Rather than expand check_methods list of arguments, it seemed easier to make check_methods a method of Methods, so that check_methods could access Methods' fields.

check_methods had an msrv parameter, which I consequently made a field of Methods. But, to avoid adding a lifetime parameter to Methods, I made the field type Option<RustcVersion> instead of the parameter's existing type, Option<&RustcVersion>. This seemed sensible since RustcVersion implements Copy. But this broke a lot of code that expected an Option<&RustcVersion> or &Option<RustcVersion>. I changed all of those occurrences to Option<RustcVersion>. IMHO, the code is better as a result of these changes, though.

The msrv-related changes are in their own commit to (hopefully) ease review.

Closes #1015

changelog: optionally allow expect and unwrap in tests

r? @llogiq

[llogiq]

Yeah, the & and .as_ref()s are noisy. I doubt it'll make much of a difference perf-wise. Thank you!

@bors r+

[bors]

📌 Commit 597f61b has been approved by llogiq

[bors]

⌛ Testing commit 597f61b with merge 4667198...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 4667198 to master...