from_u32_unchecked: check validity when debug assertions are enabled
from_u32_unchecked: check validity when debug assertions are enabled

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust