Add dashmap feature with GetSize impls for DashMap and DashSet

[AlJohri]

Summary

• Add optional dashmap Cargo feature exposing GetSize impls for dashmap::DashMap and dashmap::DashSet.
• Mirrors the std HashMap / HashSet impl in impls/collections.rs: iterate entries (under DashMap's per-shard read locks), sum each key/value's heap size, plus capacity() * size_of::<(K, V)>() for the bucket allocation.

Notes

• Neither get-size2 nor mem_dbg-rs had dashmap support before this — no upstream reference impl to cross-check against.
• default-features = false on the dashmap dep matches the convention used by the other optional deps in this crate.

Test plan

• cargo test -p get-size2 --features dashmap test_dashmap
• cargo test --all-features (36 unit + 8 derive doctests + 12 lib doctests, all green)

[bircni]

thank you! new release will come later today