std.algorithm.iteration: Fix a -dip1000 compilable issue

[carblue]

https://github.com/dlang/phobos/blob/master/dip1000.mak with
aa[std.algorithm.iteration]=-dip1000
Errors when running: make -f posix.mak std/algorithm/iteration.test
...

T=`mktemp -d /tmp/.dmd-run-test.XXXXXX` &&                                                              \
  (                                                                                                     \
    ../dmd/generated/linux/release/64/dmd -od$T -conf= -I../druntime/import  -w -de -dip25 -m64 -fPIC -transition=complex -O -release -dip1000  -main -unittest generated/linux/release/64/libphobos2.a -defaultlib= -debuglib= -L-ldl -cov -run std/algorithm/iteration.d ;     \
    RET=$? ; rm -rf $T ; exit $RET                                                                   \
  )
std/algorithm/iteration.d(4245): Error: @safe function std.algorithm.iteration.__unittest_L4227_C7 cannot call @system function std.algorithm.comparison.equal!().equal!(Result, string[]).equal
std/algorithm/iteration.d(5525): Error: static assert:  is(typeof(sum(R)(R r) if (isInputRange!R && !isInfinite!R && is(typeof(r.front + r.front)))((__error)[])) == double) is false

The remaining errors are from std/container/slist.d only

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[wilzbach]

That's not really a proper fix and we shouldn't revert to such hacks :/

[WalterBright]

Why does making it public make it safe?

[wilzbach]

I just tried this locally and if I set @trusted: to std.container.slist, I can compile + test std.algorithm.iteration successfully with -dip1000 (e.g. pmake std/algorithm/iteration.test)
-> So I suggest to drop this specific change.

[carblue]

@wilzbach Done as suggested