b3sum: download ældre versioner

version 1.8.7
Changes since 1.8.6:
- Remove the arrayref dependency. Unfortunately the crates.io owner
was compromised.

version 1.8.6
Changes since 1.8.5:
- update_mmap and update_mmap_rayon (and by extension b3sum) now
use seek rather than metadata to get the length of a file/mapping,
and they tolerate mmap failures. That means b3sum will now memory
map e.g. Linux block devices, which support mapping despite reporting
length 0 in metadata. Hashing NUL files on Windows also works now,
where previously it was an error unless you used --no-mmap or <.
This change was originally proposed by @nabijaczleweli.

Changes since 1.8.4:
- Forcibly disable LTO when compiling C intrinsics from the Rust build.
This fixes a build break on Arch Linux ARM:
#550