Update to Rust 1.49

[tjkirch]

https://blog.rust-lang.org/2020/11/19/Rust-1.48.html

Perhaps the most interesting change related to our SDK is the move to using ninja by default instead of make, which can improve build time by 35%.

rust-lang/rust#74922

[tjkirch]

Also, now 1.49: https://blog.rust-lang.org/2020/12/31/Rust-1.49.0.html