style: use strip_prefix instead of manual starts_with/slice

[yhx-12243]

strip_prefix is stable since 1.45.0, which is less than the MSRV (1.60) of this crate, and it generally generates better code.

[cuviper]

I'm surprised clippy didn't see this, but maybe it only lints if there's a manual len() too.
https://rust-lang.github.io/rust-clippy/master/#/manual_strip

Thanks!

[yhx-12243]

I'm surprised clippy didn't see this, but maybe it only lints if there's a manual len() too. https://rust-lang.github.io/rust-clippy/master/#/manual_strip

Thanks!

Yes I've already proposed at rust-lang/rust-clippy#12863. 👀