Annotate the incompatible forbid lint in algebra

[clearloop]

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---

 𝝺 rustc --version
rustc 1.50.0-nightly (c919f490b 2020-11-17)
 𝝺 cargo build
   Compiling zkp-u256 v0.2.0
error[E0453]: warn(unsafe_code) incompatible with previous forbid in same scope
  --> /Users/mercury/.cargo/registry/src/github.com-1ecc6299db9ec823/zkp-u256-0.2.0/src/lib.rs:35:5
   |
6  | #![forbid(unsafe_code)]
   |           ----------- `forbid` level set here
...
35 |     unsafe_code,
   |     ^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0453`.
error: could not compile `zkp-u256`

To learn more, run the command again with --verbose.