Add Fix128 and UFix128 types to Cadence

[SupunS]

Working towards #484

Description

Implements the FLIP: onflow/flips#342

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Cadence Benchstat comparison

This branch with compared with the base branch onflow:master commit f69e3d8
The command for i in {1..N}; do go test ./... -run=XXX -bench=. -benchmem -shuffle=on; done was used.
Bench tests were run a total of 7 times on each branch.

Collapsed results for better readability

[turbolent]

@SupunS Now that the FLIP has been merged, is this PR ready to get merged? Is there anything left to be done other than resolving the conflict?

[SupunS]

I'm cleaning up a few things (mostly errors) in onflow/fixed-point. Once that's done, need to tag a release there, and then can update this branch (will open a PR on to of this). After that this PR should be ready to get merged.

[SupunS]

This PR is now ready to be merged!

All changes in this PR were added from the below smaller PRs, and they have been reviewed already individually.

• Adding Fix128 type from this PR.
• Add more tests for Fix128 type/value  #4140
• Add UFix128 type to Cadence #4147
• Improve fixedpoint conversions #4151
• Test least-significant decimal point truncation #4155
• [Compiler] Support Fix128 and UFix128 types in compiler #4164
• Update to latest fixed-point version #4194

So reviewing this PR entirely is not really necessary, but one final look is much appreciated 🙏