Drop nim-json-serialization for testing

[mratsim]

How to lose an evening due to flaky tooling and bugs (nim-lang/RFCs#300)

Context

1. PR Double-Precision towering #155 is blocked by CI failing to clone BearSSL, a commit was removed from the tree (security issue?) https://github.com/mratsim/constantine/runs/1865343014#step:15:60
2. BearSSL is brought by a dependency chain that involves nim-json-serialization Strange dependency tree status-im/nim-json-serialization#25
3. Nimble doesn't support task-level dependencies which makes it a pain to specify packages as optional Feature: Task-level dependencies nim-lang/nimble#482, Optional dependencies nim-lang/nimble#506
4. I couldn't test Constantine as a result

Incidental Fixes

1. No more workaround for Generics regression: getTypeInst causes issue with generic constraints status-im/nim-serialization#33
   Which required uninstalling nim-serialization before reinstalling it because Nimble has no pinned dependency support Locking of dependencies nim-lang/nimble#127

   constantine/.github/workflows/ci.yml

   Lines 200 to 209 in c4a2dee

   	- name: Install test dependencies
   	shell: bash
   	# Workaround #113 and https://github.com/status-im/nim-serialization/issues/33
   	# and nimble flaky pinning / dependency resolution,
   	# json_serialization install would override nim-serialization pinning
   	run: |
   	nimble refresh
   	nimble install -y gmp stew json_serialization
   	nimble uninstall -y serialization
   	nimble install serialization@#217d78a

2. No more CI being stuck due to slow resolving of dependencies: Modern techniques for dependency resolution nim-lang/nimble#890, 40s jusst to install gmp, stew and nim-json-serialization (https://github.com/mratsim/constantine/runs/1851076249#step:15:1)
   

3. No more fighting against the generic sandwich Generics "sandwiched" between two modules don't mixin their scope symbols properly nim-lang/Nim#11225