Add `thiserror::Error` proc macro to `std::prelude`

[bjorn3]

@robinm

Would having a 48 bits prefix and 16 bits suffix (which increase the chances that each compilation unit request more than 1 prefix) help?

Probably.

@robinm

And IIRC functions are effectively using only 48 bits out of the 64, so would it be possible to trick the linker and use relocation if needed to guarantee that prefix are unique as-if it was a function address? Maybe this would require to compute the discriminant at runtime by doing &some_function_with_global_unique_adress_guaranteed_by_the_linker + unique_id_in_the_crate?

Linker tricks unfortunately don't work across dylibs. There is no way to guarantee that the dynamic linker deduplicates symbols. This is also why generic statics are not supported.