rustc: make {Closure,Generator}Substs::split as cheap as possible.
rustc: make {Closure,Generator}Substs::split as cheap as possible.

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