lang/rust: Update to 1.31.1
- Simplify .cargo-checksum.json handling . Similar to USES=cargo we disable vendor checksum checking by clearing the list of files in all .cargo-checksum.json, so that cargo does not complain when we patch something. The overall distfile is already checked by us which should be enough.
- Add an overridable CARGO_VENDOR_DIR variable to set the crate vendor directory which has moved location in nightly
- Add upstream openssl-sys patch to fix build with LibreSSL 2.9.0 
Submitted by: email@example.com , firstname.lastname@example.org 
Differential Revision: https://reviews.freebsd.org/D18610