Satisfy clippy 1.67 beta.

[xStrom]

Clippy lints addressed:

• uninlined_format_args

[xStrom]

Yeah there are even some more, which I'll also handle.

However the more serious issue is that there is an infinite loop of clippy with the beta toolchain.

warning: variables can be used directly in the `format!` string
   --> druid\src\widget\flex.rs:619:13
    |
619 | /             debug_assert!(
620 | |                 flex >= 0.0,
621 | |                 "flex value for space should be greater than equal to 0, received: {}",
622 | |                 flex
623 | |             );
    | |_____________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
    = note: `#[warn(clippy::uninlined_format_args)]` on by default

into

warning: panic message contains an unused formatting placeholder
   --> druid\src\widget\flex.rs:621:84
    |
621 |                 "flex value for space should be greater than equal to 0, received: {flex}"
    |                                                                                    ^^^^^^
    |
    = note: this message is not used as a format string when given without arguments, but will be in Rust 2021
    = note: `#[warn(non_fmt_panics)]` on by default
help: add the missing argument
    |
621 |                 "flex value for space should be greater than equal to 0, received: {flex}", ...
    |                                                                                           +++++
help: or add a "{}" format string to use the message literally
    |
621 |                 "{}", "flex value for space should be greater than equal to 0, received: {flex}"
    |                 +++++

I will investigate it tomorrow. Perhaps just updating the edition will solve the issue. However I feel clippy shouldn't recommend fixes that it itself will immediately complain about.

[xStrom]

Okay, so with the beta clippy both assert! and debug_assert! are incorrectly handled with Rust 2018. This was already fixed in rust-clippy#10055 and has landed in nightly, but it's unclear to me if it will arrive with stable 1.67 or some future version.

I think we might be able to just upgrade to Rust 2021 and get around this issue. I'll see if I can make it happen.

[xStrom]

Alright, this is ready for another look. It solves all clippy errors now.