Update contract Wasm runtime interface to support multi token

[xlc]

Update contract Wasm runtime interface to support multi token by making existing one defaults to CENTRAPAY and add new one comes with asset id argument

• Add a DefaultAssetId type to contract

• Add TryFrom<u64> and Into<u64> to AssetId in MultiCurrency trait (depends on Multi currency trait design #5)

This assumes AssetId cannot be larger than u64. We can revisit later to see if we need to drop this assumption.

• All existing runtime interfaces use DefaultAssetId

Runtime interfaces:

plug-blockchain/srml/contract/src/wasm/runtime.rs

Line 226 in dd8a778

define_env!(Env, <E: Ext>,

• New ext_asset_balance(ctx, asset_id: u64) to query GA balance
• New ext_asset_transfer(ctx, asset_id: u64)

Later will need to modify ext_call and ext_create to be able to transfer multiple tokens during contract call or deployment.

Meta issue: #13