Rename `inline_fluent!` to `msg!` This was the most popular name for the macro as voted in [#t-compiler > Bikeshed the new `inline_fluent!` macro @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Bikeshed.20the.20new.20.60inline_fluent!.60.20macro/near/572751863)