Refactor `AssetId` in pallet-asset-index as enumeration

[clearloop]

Issue summary

one question about the From restriction, was this necessary for the benchmarks?
I don't think we should necessarily restrict it to be an u32, for example it could be end up something like Acala's currencyId

https://github.com/AcalaNetwork/Acala/blob/master/primitives/src/currency.rs#L208-L212

Other information and links

• feat(benchmarks): add benchmarks and specific weights to all pallets #91 (review)
• Use PINT or IndexToken in our code documentation? #78