sapphire-contracts: Optimize gas usage where possible

[matevz]

SUMMARY

@peternose warned us that using sapphire-contracts in Remix yields a number of warnings about possible gas optimizations.

Figure out the correct tooling to benchmark how much gas Sapphire.sol and SIWE calls require and if there are any obvious gas improvements possible.

ISSUE TYPE

• Feature Idea

COMPONENT NAME

sapphire-contracts