lang/rust: Update to 1.31.1

• Simplify .cargo-checksum.json handling [1]. 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 [2]

Changes: https://blog.rust-lang.org/2018/12/20/Rust-1.31.1.html
Submitted by: mikael.urankar@gmail.com [1], ml@vishwin.info [2]
Differential Revision: https://reviews.freebsd.org/D18610