Upgrade to rust 1.38

[mvines]

No description provided.

[mvines]

Blocked on dealing with https://rust-lang.github.io/rust-clippy/master/index.html#try_err

I mostly just want to suppress this lint: return Err("failed".into()); is not more readable than Err("failed")?;

[garious]

I read return Err("failed".into()); as "return an error". I read Err("failed")? as "that clever idiom that returns an error after hoping the compiler will apply const_prop and optimize the Err passed to try_err into return Err or otherwise add runtime overhead that may or may not be optimized away by the CPU's branch prediction".

[mvines]

586e0a6 Suppress nighly safety_doc warning
5aab286 Rebuild SVGs
a20f128 Upgrade to rust 1.38