Make `rust.overflow-checks-std`option default to `rust.overflow-checks`.
Make `rust.overflow-checks-std`option default to `rust.overflow-checks`.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust