Fix MSRV of the macros to actually be 1.46

[FSMaxB]

Turns out that panic! is only stable in const contexts starting with Rust 1.57. Initially I was going to introduce a dual MSRV again with 1.57 required for the macros, but then I remembered that there was a trick to get compile time assertions with previous versions of rust.

The code in the macros is far from elegant and doesn't produce great compiler errors, but it does work on 1.46.

This change together with #41 also finally allows running the full tests on our MSRV.

[FSMaxB]

NOTE: This feature wasn't supported in cargo for 1.46, but it still serves as a machine readable way to specify the MSRV, which is why I added it here.

[jplatte]

How did you notice the macros didn't work on MSRV?

[FSMaxB]

How did you notice the macros didn't work on MSRV?

cargo +1.46 test --all-features

[FSMaxB]

But now we have the full tests in CI, so it shouldn't happen again.

[jplatte]

Okay, so you actually just went to improve CI, this wasn't in a real project?

I'm wondering at which point we should just increase the MSRV ^^

[FSMaxB]

Yeah, not a real project.

I was thinking about ways to come up with a policy for determining the MSRV and the only thing I came up with was looking at some target platforms that ship rustc.

For example looking at Debian versions. Oldstable has 1.48 for example.