This replaces the link meta attributes with a pkgid attribute and uses a hash
of this as the crate hash. This makes the crate hash computable by things
other than the Rust compiler. It also switches the hash function ot SHA1 since
that is much more likely to be available in shell, Python, etc than SipHash.
Fixes#10188, #8523.
It was a little hard for me to believe, but it seems that re-exporting
an impl doesn't work at a, because encoder::encode_info_for_mod requires
that all the impls in the current module's impl map be local (that is,
bound to a value in the current crate's item map). Fixed it.
Closes#2414.
