Improvements of chain extension implementation

[kvinwang]

This PR add a attribute_macro chain_extension as a drop-in replacement of ink_lang::chain_extension.
The macro generates some extra codes:

• A trait PinkExtBackend for the runtime to implement.
• Const variables for the function ids.
• A call dispatcher

With this improvement, the runtime now implements the extension functions more type-safe.

[h4x3rotab]

Are we going to put the context of the current contract to a global variable?

[kvinwang]

Are we going to put the context of the current contract to a global variable?

No. It might be impossible because the current contract can be changed without notifying the pink runtime.

[h4x3rotab]

Finally understood how it works, haha