Fix return types of context::contract_id & context::msg_asset_id

[nfurfaro]

2 functions in the context module currently return a b256 type as the ContractId type didn't exist when they were written.

• msg_asset_id()
• contract_id
  This fixes that, and ports the tests from the sway repo into the std-lib repo, using the rust SDK.
  I also modified the second function msg_asset_id() to reuse the first for consistency.

This closes #45

Note: still some failing tests, blocked by an issue related to the difference between Return and ReturnData and how the SDK reads returned data. @digorithm is already on it.

[digorithm]

The issue mentioned on the SDK side has been fixed here: FuelLabs/fuels-rs#112

Just gotta wait for some reviews and a release. Will keep you updated!

[nfurfaro]

Just a small change here.
A follow-up PR will be porting the context tests over to the new harness and provides good test coverage for this. It's WIP already, but blocked atm.

Note: this is a breaking change, so our next release will have to reflect that.