std.container.slist: Fix -dip1000 compilable issues

[carblue]

A new attempt since #6295

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

T=`mktemp -d /tmp/.dmd-run-test.XXXXXX` &&                                                              \
  (                                                                                                     \
    ../dmd/generated/linux/release/64/dmd -od$T -conf= -I../druntime/import  -w -de -dip25 -m64 -fPIC -transition=complex -O -release -dip1000 -version=DIP1000  -main -unittest generated/linux/release/64/libphobos2.a -defaultlib= -debuglib= -L-ldl -cov -run std/container/slist.d ;     \
    RET=$? ; rm -rf $T ; exit $RET                                                                   \
  )
std/container/slist.d(280): Error: @safe function std.container.slist.SList!int.SList.__unittest_L278_C11 cannot call @system constructor std.container.slist.SList!int.SList.__ctor!int.this
std/container/slist.d(282): Error: @safe function std.container.slist.SList!int.SList.__unittest_L278_C11 cannot call @system constructor std.container.slist.SList!int.SList.__ctor!int.this
std/container/slist.d(27): Error: template instance `std.container.slist.SList!int` error instantiating
std/container/slist.d(801): Error: template instance `std.container.slist.SList!string` error instantiating

Changing to:
size_t insertFront(Stuff)(scope Stuff stuff)
if (isInputRange!Stuff && isImplicitlyConvertible!(ElementType!Stuff, T))

compiles everything except: auto sl = SList!string(["a", "b", "d"]);

std/container/slist.d(365): Error: scope variable item may not be copied into allocated memory
std/container/slist.d(154): Error: template instance `std.container.slist.SList!string.SList.insertFront!(Range)` error instantiating
std/container/slist.d(250):        instantiated from here: __ctor!(Range)
std/container/slist.d(801):        instantiated from here: SList!string

Thus I'm going to try with a dedicated constructor (or ?) for the string types, probably including dup-ing consructor arguments. We'll see.

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Testing this PR locally

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[wilzbach]

Didn't we agree that this is bad because we duplicate memory without the user explicitly agreeing too.
Why not version(DIP1000) else out the test for now? And/or report the DMD error I reduced here: #6295 (comment)

[dnadlinger]

Also, wouldn't special-casing strings this way break all other reference types anyway?

[dnadlinger]

Also, wouldn't special-casing strings this way break all other reference types anyway?

[PetarKirov]

Inserting an element in a container is really escaping it, so this function can't accept scope arguments by design, unless it always stores a copy of them.