`lib`: alloc feature flag

[github-actions]

On 2023-07-27 @prestwich wrote in e34200e “Merge pull request #274 from DaniPopes/no_std2”:

alloc feature flag

// Workaround for proc-macro `uint!` in this crate.
// See <https://github.com/rust-lang/rust/pull/55275>
extern crate self as ruint;

// TODO: alloc feature flag
#[macro_use]
extern crate alloc;

#[macro_use]
mod macros;

From src/lib.rs:30