Fix return types of ez80_builtin functions, optimize some implementat… #227

Job	Run time
docs	3m 1s
build-and-test (ubuntu-22.04, YES)	4m 0s
build-and-test (windows-latest, NO)	5m 25s
build-and-test (macos-14, YES)	3m 16s
build-and-test (macos-13, NO)	7m 58s
build-and-test (macos-13, YES)	5m 58s
build-and-test (macos-14, NO)	4m 29s
build-and-test (ubuntu-22.04, NO)	3m 49s
build-and-test (windows-latest, YES)	5m 27s
	43m 23s

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