Remove Serial::split #1081

Job	Run time
Check (stm32f301x6)	37s
Build Testsuite	37s
Check (stm32f318x8)	34s
Check Minimal Dependency Versions	42s
Check Documentation Build	40s
Check (stm32f302x6)	35s
Check minimal feature set	29s
Check MSRV	4m 14s
Clippy	38s
Check (stm32f302xb)	35s
Check Codegen	22s
Check (stm32f302xd)	36s
Rustfmt	16s
Cargo Deny	18s
Check (stm32f303x6)	35s
Check (stm32f303xb)	36s
Link Checker	16s
Markdown Lint	9s
Check (stm32f303xc)	36s
Check (stm32f303xd)	37s
Check (stm32f328x8)	34s
Check (stm32f358xc)	42s
Check (stm32f398xe)	36s
Check (stm32f373x8)	33s
Check (stm32f378xc)	33s
Check (stm32f334x4)	38s
	17m 38s

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