refactor(name): simplify `uninit_u8_array`

[SandaruKasa]

The codegen seems to be OK: https://godbolt.org/z/da5Psn1s9

• Just as good (noop) on -O1 & above.
• Even smaller (and with just 1 memset instead of 3 memcpy's) on -O0.

Bonus point: no need for unsafe (because, as everyone knows, unsafe=bad, Rust=good, hehe).