Rename unroll macro to unroll_n to avoid clash with nightly builtin

[OriPekelman]

Recent nightly rustc adds an unroll builtin attribute, which makes the pub(crate) use unroll; re-export ambiguous (E0659) and breaks the build on nightly. Rename the macro and its uses.

error[E0659]: unroll is ambiguous
    --> src/util/unroll.rs:17:16
     |
  17 | pub(crate) use unroll;
     |                ^^^^^^ ambiguous name
     = note: ambiguous because of a name conflict with a builtin attribute
     = note: unroll could refer to a built-in attribute

Nightly added an unroll builtin attribute, which now shadows the crate's
unroll macro. Stable still builds fine. Present on master and the dev_2.0*
branches.

Fix: rename the macro (e.g. unroll → unroll_n), or otherwise disambiguate
(raw identifier r#unroll, or path-qualify the re-export).