chain/owasm: Increase wasm gas limit and consume gas when reading / writing

[sorawit]

Implementation details

This PR bumps the Wasm gas limit of Prepare to 1M and Execute to 5M. Reason being that generally, a wasm opcode on average takes a few nanosecs to run. From a quick experiment on my local machine, 5M gas takes ~10ms. This is in line with what other projects, such as Near (1 Wasm gas = 3856371gas = ~3.85ns) or CosmWasm (100 Wasm gas = 1 SDK gas) is doing at the moment.

Additionally, to prevent attackers repeated calling into heavy OEI operation (such as read calldata), the PR also makes it consume gas equal to span size. This should generally not be an issue for normal users as span size is ~1024. Even 100 reads still only cost 100k gas.

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