Revert "Use multiple codegen units"
Revert "Use multiple codegen units"

This reverts commit 03acf67a0e3a2780d2a1443f4e4596b69d9a2b09.

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