Support expressions for extension and function during chain extension definition

[xgreenx]

Both extension and function attributes require an explicit literal argument, which makes it impossible to have these constants defined in a single place and refer to them from both the trait definition and the runtime-side.

It would be more usable if ink! allows usage of the constant expression during the definition of the chain extension. It should be possible todo because ink! doesn't have any specific logic around literal values.