libstd: Fix BigUint::is_even
auto merge of #6429 : gifnksm/rust/bigint-is_even, r=catamorphism

`BigUint::is_even()` didn't return correct value.
rustpkg: Remove #[allow(vecs_implicitly_copyable)]
compiletest: Remove #[allow(vecs_implicitly_copyable)]
syntax: Remove #[allow(vecs_implicitly_copyable)]
Remove the last instance of vecs_implicitly_copyable
Remove vecs_implicitly_copyable from the compiler