Slices, text and bytes, and moving out of raw memory

Decided

D-0050, settled by the language’s author: part of spec §18. Older text cites it as 18.50.

xs[a..b] (and xs[a..], xs[..b]) slices an array or slice with checked ends, and a pointer inside unsafe with its end written — chosen over an unsafe slice(ptr, len) builtin, since the range form also serves checked subslicing. s.len and s.ptr on a str; s as []u8 is safe; str::from_utf8(bytes) -> str? is the checked conversion, and bytes as str and p as cstr the unsafe, unchecked ones — the user asked for the safe way to exist, not only the fast one. IndexMut beside Index, chosen when the container is writable. move_out(dst) completes construct/destroy: a container cannot return an element by value without it (§5.3, §11.2, §14.3)