change binding name of for loop lowering to appease clippy
change binding name of for loop lowering to appease clippy

With the latest change to for loop lowering, a `_next` binding was introduced.
Unfortunately, this disturbs clippy's `used_underscore_binding` lint. This
commit just renames the binding to `__next` so clippy will be happy. It should
have no other effect.

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