refactor: use ::core instead of core

[SteveLauC]

The issue I encountered

In my project, I have a module named core, it clashes with the Rust core within the ctor macro, as you can see from the error:

$ ls -l src
Permissions Links Size User  Group Date Modified Name
.rw-r--r--      1    0 steve steve  8 Dec 19:57  core.rs
.rw-r--r--      1   79 steve steve  8 Dec 19:57  main.rs

$ cat src/main.rs
mod core;

use ctor::ctor;

#[ctor]
static INT: i32 = {
    1
};

fn main() {}

$ cargo b -q
error[E0433]: failed to resolve: could not find `marker` in `core`
 --> src/main.rs:5:1
  |
5 | #[ctor]
  | ^^^^^^^ could not find `marker` in `core`
  |
  = note: this error originates in the attribute macro `ctor` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing one of these items
  |
3 + use core::marker;
  |
3 + use std::marker;
  |

error[E0433]: failed to resolve: could not find `ops` in `core`
 --> src/main.rs:5:1
  |
5 | #[ctor]
  | ^^^^^^^ could not find `ops` in `core`
  |
  = note: this error originates in the attribute macro `ctor` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing one of these items
  |
3 + use core::ops;
  |
3 + use std::ops;
  |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `rust` (bin "rust") due to 2 previous errors

How to fix it

Add :: before core so that it will use the external core module, i.e., the Rust core (not the core from my project)

$ cat Cargo.toml | grep ctor
ctor = { git = "https://github.com/SteveLauC/rust-ctor", branch = "core" }

$ cargo b -q

[mmastrac]

Released in ctor v0.2.6