This PR implements the gas limit API, returning the maximum ref_time per
block. Solidity contracts only know a single weight dimension and can
use this method to get the block ref_time limit.

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Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <cyrill@parity.io>
Co-authored-by: command-bot <>