Integrate `hashtree` for better hashing performance

[arnetheduck]

https://github.com/status-im/nim-ssz-serialization/pull/35 brings in https://github.com/prysmaticlabs/hashtree as a supported backend for SHA256, giving a nice little performance boost to all hash_tree_root calls on supported platforms / compilers.

Expected gains are on the order of 30% which in the case of a replayed state nets us 0.2-0.3s improvement.

More about this design here: https://hackmd.io/@potuz/BJyrx9DOF - kudos to @potuz for this excellent library!

[github-actions[bot]]

Unit Test Results

         9 files  ±0    1 115 suites  ±0   23m 52s :stopwatch: - 1m 8s   4 245 tests ±0    3 898 :heavy_check_mark: ±0  347 :zzz: ±0  0 :x: ±0  16 929 runs  ±0  16 531 :heavy_check_mark: ±0  398 :zzz: ±0  0 :x: ±0 

Results for commit 269e13b2. ± Comparison against base commit 109007dc.

:recycle: This comment has been updated with latest results.

[tersec]

fatal: remote error: upload-pack: not our ref dc1992a73be5643cf4b06bd29a22478f8d32513b
Errors during submodule fetch:
	vendor/nim-ssz-serialization

probably that's not available in the git repo, but this error triggers regardless on git pull in such cases, even when not having checked out this branch specifically