Support Timepoint, Duration

[jayz22]

What

Resolves #724

• Implement {Timepoint, Duration} <-> u64 conversions
• A drive-by clean up for get_current_call_stack to return Address
• Clean up some trailing whitespaces in env.json

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]

[leighmcculloch]

What's the win out of having these two disjoint types when they're just u64 values?

I think I missed the conversation when we added them.

[jayz22]

What's the win out of having these two disjoint types when they're just u64 values?

I think I missed the conversation when we added them.

They are special case u64 subtypes that can have different semantics and operations. They can represent for example ledger timestamp and can specify its own display format (instead of a plain u64). Similar to how String, Symbol are related to Bytes.
For now we just support them by converting them to and from u64, which is the bare minimum so that the guest can implement the semantics in ways they want. We might enrich them post V1.