Feat: upgrade asset system

[SupremoUGH]

Third part of #153. It consists of the changes related to the asset system. To be merged after #282.

Changes:
manta-util:

• codec.rs: Decode implementation for u128.

manta-accounting:

• asset.rs: Upgrade to the new asset system. Now Asset is more general and depends on two generics.
• transfer: Included the associated types AssetId and AssetValue in the Configuration trait.
• rest of manta-accounting: Minimal changes so it compiles.

manta-pay:

• config: Concrete implementations of AssetId and AssetValue for Config, with the relevant trait implementations.
• rest of manta-pay: minimal changes so it compiles.

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Before we can merge this PR, please make sure that all the following items have been checked off:

• Linked to an issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Added one line describing your change in CHANGELOG.md and added the appropriate changelog label to the PR.
• Re-reviewed Files changed in the GitHub PR explorer.
• Checked that changes and commits conform to the standards outlined in CONTRIBUTING.md.

[GhostOfGauss]

@bhgomes do you know why the CI Lint check is skipping?

[bhgomes]

@bhgomes do you know why the CI Lint check is skipping?

Not really sure why, will have to investigate. Probably it's timing out or overusing resources.