Tweak assumeSafeAppend documentation

Avoids using pronouns. `s/could/will/` in `those elements could be overwritten`.
making assumeSafeAppend return a value.
simpler unittest with basic usage
make assumeSafeAppend return its array
Update object_.d
space after //
use auto ref for assumeSafeAppend
"Workaround" 10574
"Workaround" 10574
Merge pull request #540 from monarchdodra/assumeSafeAppend

assumeSafeAppend improvement
Typo in core.simd, market -> marked
Merge pull request #546 from Dav1dde/patch-2

Typo in core.simd, market -> marked