[Sol->Yul] Support exponentiation in constants

[axic]

The following should compile:

    uint constant DEPOSIT_CONTRACT_TREE_DEPTH = 32;
    // NOTE: this also ensures `deposit_count` will fit into 64-bits
    uint constant MAX_DEPOSIT_COUNT = 2**DEPOSIT_CONTRACT_TREE_DEPTH - 1;

This previously attempted by #8945.

Closes #8853.

[chriseth]

I don't think this is different from regular 'exp'.

[axic]

It is not, unless we would have constant expressions. Is there a tracking issue for support exp in sol-yul?

[chriseth]

The value of the constant is just treated as a regular expression: https://github.com/ethereum/solidity/blob/develop/libsolidity/codegen/ir/IRGeneratorForStatements.cpp#L196