Increase max ram to 64 MiB

[adlerjohn]

No description provided.

[vlopes11]

Looks fine, but shouldn't we increase the maximum mem access as well? As long as the user don't overflow the available memory and pays enough gas, I don't see why to restrict it to a small fraction of the VM memory

[adlerjohn]

We should, independently. But that needs to be reworked to differentiate between memory that must be accessed atomically (i.e. hash functions) vs memory that can be accessed one word at a time (i.e. memory clear/copy/eq). So separate PR.