Add "typechecking" to DynamicLib

[kmcallister]

We'd produce a metadata table with a TypeId hash for each exported symbol and check it in DynamicLibrary::symbol. The function will still need to be unsafe because there are plenty of ways to deliberately circumvent the check (collisions of the 64-bit hash, hex-editing the .so file, etc) but it will fix the poor user experience of accidental type mismatch when using plugins (in the case of rustc plugins, this is #14841).

We'll also have DynamicLibrary::raw_symbol which does no typecheck, gives an *mut u8, and can be used with non-Rust .so's.

I plan to work on this after #8157.

[huonw]

Maybe DynamicLibrary could be kept separate and a higher-level one built in top? (E.g. DynamicLibrary & RustDynamicLibrary or CDynamicLibrary & DynamicLibrary.)

E.g. theoretically RustDynamicLibrary could check for validity/that's it's a rust library/read metadata when opening it, while this is not possible wit a non-Rust .so.

[kmcallister]

Possibly over-engineered, but how about

pub struct DynamicLibrary<E> {
    extra: E,
    …
}

impl<E> DynamicLibrary<E> {
    …
}

pub struct RustLibrary {
    …
}

impl DynamicLibrary<RustLibrary> {
    …
}

[alexcrichton]

This is unfortunately fairly unlikely to happen, and we're unlikely to stabilize DynamicLibrary as-is, so I'm going to close this as we're not tracking feature requests for various libs in this issue tracker.