Support for 64-bit counters was a hack built upon the version with
a 32-bit counter, that emulated a larger counter by splitting the
input into large blocks.
This is fragile, particularily if the initial counter is set to
a non-default value and if we have parallelism.
Simply add a comptime parameter to check if we have a 32 bit or a
64 bit counter instead.
Also convert a couple while() loops to for(), and change @panic()
to @compileError().
057d30bacc