Proposal: add `export-symbols` feature flag

[folkertdev]

We currently have a number of functions like this in our public api

#[export_name = prefix!(inflate)]
pub unsafe extern "C-unwind" fn inflate(/* ... */) -> i32 {
    /* ... */
}

The prefix! macro can be used to namespace the symbols that we export. Recently, a new idea came up, we could export the symbols only conditionally:

#[cfg_attr(feature = "export-symbols", export_name = prefix!(inflate))]
pub unsafe extern "C-unwind" fn inflate(/* ... */) -> i32 {
    /* ... */
}

That solves a common problem with using zlib-rs in existing projects: the names we export clash with another (usually indirect) dependency that also defines these symbols (e.g. openssl relies on libz-sys which defines these symbols). It would also be possible to have multiple versions of zlib-rs in the same dependency tree (that has no practical value, but the flexibility could be useful).

Proposal

We add the export-symbols feature flag

export-symbols = []

And use cfg_attr to only export the symbol names when this feature flag is active.

The export-symbols feature flag is off by default: when used as a rust crate, the symbols will not be exported!

Because that changes existing behavior, I think this needs to be a major version bump.