ci: integrate callgrind benchmarks using iai-callgrind

[Zul-Qarnain]

This PR adds instruction-level benchmarks using iai-callgrind and integrates them into CI to catch regressions deterministically:

• Adds benches/callgrind.rs testing core vector operations (push, pop, insert/remove, slice conversions, drain, retain_mut, into_iter) across both inline and spilled heap storage.
• Adds a callgrind job to .github/workflows/checks.yml running the benchmarks on both default features and --all-features.

[alejandro-vaz]

CI shows nothing

callgrind::push_group::bench_smallvec_push_inline
  Instructions:                           0|N/A                  (*********)
  L1 Hits:                                0|N/A                  (*********)
  L2 Hits:                                0|N/A                  (*********)
  RAM Hits:                               0|N/A                  (*********)
  Total read+write:                       0|N/A                  (*********)
  Estimated Cycles:                       0|N/A                  (*********)

[alejandro-vaz]

it doesn't seem to be working properly