Remove Serial::split #1072

Job	Run time
Check (stm32f301x6)	35s
Check (stm32f318x8)	34s
Check Minimal Dependency Versions	39s
Check Documentation Build	37s
Build Testsuite	37s
Check (stm32f302x6)	33s
Check (stm32f302xb)	38s
Check minimal feature set	30s
Check MSRV	4m 16s
Check (stm32f302xd)	35s
Clippy	35s
Check (stm32f303x6)	38s
Check Codegen	23s
Check (stm32f303xb)	35s
Markdown Lint	12s
Rustfmt	15s
Check (stm32f303xc)	34s
Check (stm32f303xd)	39s
Cargo Deny	18s
Link Checker	10s
Check (stm32f328x8)	34s
Check (stm32f358xc)	37s
Check (stm32f398xe)	33s
Check (stm32f373x8)	32s
Check (stm32f378xc)	33s
Check (stm32f334x4)	37s
	17m 19s

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