[Rust] Vulnerability in dependency: smallvec

[drtychai]

The smallvec crate should be updated to v1.6.1 or soundness of a v1.0 patch must be implemented+validated.

See:

• relevant issue
• relevant pr
• RustSec Advisory (disclosed January 8, 2021)

[mbrubeck]

Note that the current smallvec = "1.0" requirement is compatible with any 1.x.y version of smallvec, so existing projects that depend on the flatbuffers crate can apply the fix simply by running cargo update. New projects will use the fixed version by default. No changes to the flatbuffers crate are required.