Basic functionality (and initial `repro` crate)

[tarcieri]

Adds extremely basic functionality for invoking cargo build --locked as cargo repro build.

Though this may seem fairly pointless in and of itself, the goal of a followup commit would be to collect environmental information during this step (OS/release, rustc/cargo version, CWD, environment variables, git commit, C/C++ compiler versions if applicable) and use that during the verification process to detect and highlight mismatches.

This commit attempts to split the CLI app (i.e. cargo-repro) from a library-level crate containing the core functionality (ala cargo-audit and the rustsec crate), in case there is interest in driving these sorts of builds from external tooling.