Minor readability fixes

[Plecra]

I needed the crate for another project and spotted a couple lines that could be simplified. The code should behave the same

[Lokathor]

Does putting the initial const outside the macro cause issues if you use the macro twice in the same scope?

[Plecra]

I doubt it... I'll add a test

[Plecra]

And in case I misunderstood - the mangled __UTF16_LIT_PREFIX_THAT_SHOULD_NEVER_CLASH_WITH_OUTER_SCOPE_UTF8 name is still in a small, internal scope. The actual logic is now two layers deep

[Plecra]

Ah, CI is failing on the checked_sub not being stable in 1.46.0. Bummer. I'll revert that