Methods request for CKB precompiled contract

[fjchen7]

Propose-a-new-feature

For now CKB precompiled contract only provides method getCell(bytes32 txHash, uint32 index) for other contract to use (see here). We probably need more methods for proof verification in IBC, for example

• getLiveCells(Height height)
• getLatestHeight

The specific methods we need are not confirmed until CKB hardfork has been launched. This issue is for track and discussion.

cc @ashuralyk