[codegen] Test and document codegen for SplitAt and Split#3111

jswrenn merged 1 commit intomaingoogle/zerocopy:mainfrom

G09ea59a47925616837b859b2c3cdf7f0f919232egoogle/zerocopy:G09ea59a47925616837b859b2c3cdf7f0f919232eCopy head branch name to clipboard