Replace BnStrCompatible with AsCStr trait

[mkrasnitski]

This is a rather large PR, but the essence of the changes is in string.rs. Replaces the BnStrCompatible trait with an AsCStr trait with the following definition:

pub trait AsCStr {
    fn as_cstr(&self) -> Cow<'_, CStr>;
}

Note that this trait borrows &self instead of consuming self. This simplifies the list of implementing types and reduces the need for cloning. The as_cstr method returns a Cow in order to provide implementations both for already null-terminated strings (BnString) and for non-terminated strings (&str/String).

Note that it is still possible in some cases to create a pointer to a temporary that gets immediately dropped if calling s.as_cstr().as_ptr() inline, same as with BnStrCompatible. I was careful to avoid doing so when swapping out the traits.