EVM: use u64 for gas_limit representation for uniformity

Context

Fixes #5487 (closed).

The reason for that is not arbitrary. It's mainly because gas_limit representation in EvmHandler/Gasometer is a u64. To avoid unsafe conversion from U256 to u64, it seems to be the right approach to homogenize the type through all structures

Manually testing the MR

> make -f kernels.mk evm_kernel.wasm

Checklist

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