Add more const fn functions

[lrazovic]

This PR aims to convert all the possibles functions to const fn in order to be used in other const fn environments, but also includes several changes to cleanup the code. I have also updated assert_matches to the latest version and replaced the deprecated merge_imports with imports_granularity in the rustfmt.toml.

As a downside, these changes bumps the MSRV to the 1.46.0.

[svartalf]

Looks great, thank you!

I see that CI fails, because it is not compatible with current MSRV policy (rustc 1.31+), so it will require a new major version release, which is I'm definitely up to (would be nice to include #3 too then).
Do we now what new minimal rustc version would be required now?

[lrazovic]

The features #![feature(const_if_match)] and #![feature(const_loop)] were stabilized in rustc 1.46.0 (04488afe3 2020-08-24). [1, 2, 3]. The MSRV policy then will have to change to rustc 1.46+.

I'm going to read the discussion in #3 and I will do what I can to implement it.
Perhaps this approach would help? What do you think? serde-rs/serde#2001

[svartalf]

The MSRV policy then will have to change to rustc 1.46+.

Good to know, thank you. I'm totally okay with bumping it as long as we understand to what exactly it should be bumped :)

and I will do what I can to implement it.

I'm sorry, I didn't meant that you "should" do it, I was just pointing out that it would be great to include that PR into next release too. Nevertheless, tweaking serde implementation should be pretty straightforward, but it is still unclear to me what to do with fmt::Display implementation, as mentioned in #3. If you have any opinion on it, I would love to hear it