Expose `type_name()` method on Any.

[rickvanprim]

In a similar spirit to #61533, it would be nice if the Any trait contained an auto implemented method akin to:

fn type_name(&self) -> &'static str
{
    std::any::type_name::<T>()
}

This way it would be a lot easier print and surface errors like accidentally trying to cast Box<T> to T because &Box<T> is also &Any.

[steffahn]

Something like this would be just as useful on Error. Would’ve been a huge help for this problem posted on URLO.

[eddyb]

(cross-linking) TypeId containing a mangled name would allow this more efficiently (in terms of binary size) than via std::any::type_name, and is arguably necessary for sound TypeId (at least one some platforms?): #77125 (comment)

[eddyb]

Update: finally opened #95845 with a (v0) mangling-based TypeId representation (which could eventually be used to allow runtime demangling of type names from TypeId and therefore Any).