Intradoc links are broken when building with no default features

bstr has errors building documentation when doing so with --no-default-features.

$ cargo doc --no-default-features
warning: unresolved link to `ByteVec::unescape_bytes`
    --> src/ext_slice.rs:2783:40
     |
2783 |     /// The dual of this function is [`ByteVec::unescape_bytes`].
     |                                        ^^^^^^^^^^^^^^^^^^^^^^^ no item named `ByteVec` in scope
     |
     = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: unresolved link to `BString`
    --> src/ext_slice.rs:2786:42
     |
2786 |     /// implementation on [`BStr`] and [`BString`]. The `Debug` implementations
     |                                          ^^^^^^^ no item named `BString` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: `bstr` (lib doc) generated 2 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s

Additionally, the vast majority of intra-doc linking in this crate is hardcoding links to HTML pages that rustdoc generates. Linking in this way prevents rustdoc from properly linting whether the items being referenced exist (which is especially nice in the presence of conditional compilation).

https://github.com/BurntSushi/bstr/blob/b3cab1905c46ad7de78a032a61eef0437ed7fb58/src/lib.rs#L338-L354

Example links that are broken are this link to BString on the BStr type docs:

I've found adding this env var to CI and building the docs as part of the CI steps that test the feature matrix to be useful in preventing regressions here:

RUSTDOCFLAGS="-D warnings -D rustdoc::broken_intra_doc_links --cfg docsrs"

Start by running cargo doc --no-default-features with the reported RUSTDOCFLAGS to reproduce the warnings in src/ext_slice.rs. Review the hardcoded HTML links in src/lib.rs around lines 338-354 and the related documentation links, then update the links so rustdoc can validate them across feature configurations. Done means the feature-matrix documentation build produces no broken intra-doc link warnings.