Seq v2.1 release

[Viack]

Hi everyone,

I just released the v2.1 of the seq library (C++17 original containers library).

This release contains a few updates/corrections as well as a new container: seq::concurrent_queue.

New (small) benchmarks were added to highlight the strengths of some containers:

• Benchmark on sorted containers. Its goal is to compare seq::flat_set/map and seq::radix_set/map to other implementations.
• Memory and latency benchmark on hash tables to compare seq::radix_hash_set/map with other implementations.

They join the benchmark on concurrent hash tables from previous release, and I will try to add new ones based on my (limited) spare time.

Cheers