EVM memory extend is very slow.

[jangko]

nimbus-eth1/nimbus/evm/memory.nim

Line 28 in cd78458

proc extend*(memory: var Memory; startPos: Natural; size: Natural) =

This proc performance is very bad, especially concat operation contribute to the majority of time spent in EVM.

[mratsim]

Seems like an easy optimization, replace all concat(repeat(0.byte, foo)) patterns by setLen.
This should reduce cost by atleast 2, probably 3 by avoiding all the intermediate allocations and more if it creates memory churn in the allocator.