Fix indexed ndslice increment and decrement safety checks on GDC

[jmh530]

The safety-check helper for indexed unary operations returns auto ref from a value passed by value. For ubyte and ushort, optimized GDC 14 and 15 diagnose this as returning a reference to a local parameter, causing builds with -Werror to fail.

Change the helper to return void and discard the expression result. It still checks that the operation is @safe; the actual indexed operation and its reference-return behavior remain unchanged.

Add tests covering increment, decrement, and assignment through the returned reference for ubyte, ushort, uint, and ulong.

Validation:

• Reproduced the original warning on GDC 14.2 and 15.2.
• The focused test passes optimized compilation with -Wall -Werror and runs successfully on both GDC versions.
• Optimized ndslice unittests pass on DMD 2.113 and LDC 1.41: 19 modules each.

Fixes #483.

[jmh530]

Only failure is Circle CI, which is unrelated to this.