perf: optimize OpamString.split #3589

Job	Run time
Analyse	6s
Hygiene	24s
Build-macOS (4.14.1)	2m 28s
Cygwin	6s
Build-Linux (4.08.1)	1m 55s
Build-Linux (4.09.1)	2m 2s
Test-macOS (4.14.1)	26m 5s
Build-Linux (4.10.2)	1m 51s
Build-Linux (4.11.2)	2m 7s
Build-Linux (4.12.1)	2m 5s
Build-Linux (4.13.1)	2m 5s
Build-Linux (5.0.0)	2m 2s
Build-Linux (5.1.1)	2m 0s
Build-Linux (4.14.1)	2m 18s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	26m 37s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	39m 2s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	36m 53s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	37m 34s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	43m 46s
Cold-Linux	4m 49s
Upgrade-Linux (4.14.1)	1m 19s
Solvers-Linux (z3, 4.14.1)	52s
Test-Linux (4.14.1)	13m 30s
Solvers-Linux (0install, 4.14.1)	1m 32s
Upgrade-macOS (4.14.1)	1m 38s
Solvers-macOS (z3, 4.14.1)	1m 40s
Solvers-macOS (0install, 4.14.1)	1m 14s
	4h 18m 0s

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