Better struct handling in code generation util

[webthethird]

Generates better code, which will compile without manual editing, when dealing with structs.

• Avoids using a struct's canonical name, which will not match the generated interface once an I is prepended to the contract name.
• Handles arrays of structs in function parameters and returns
• Handles nested mappings inside and outside of structs
• Handles structs defined in other contracts/libraries
• Converts the type of enums used in structs, in case include_enums is false

[webthethird]

@montyly @0xalpharush pip-audit may need to be updated again

[webthethird]

One unresolved issue (edit: resolved below) is that structs defined in a library and referenced in a using-for statement do not get included in the interface, though they may be referenced in a function's arguments or returns. For example:
The contract at 0xaec84eac74981ab22905919cb282b78c7ca782df uses FixedLib.Position in a couple of public mappings:

contract Market is Initializable, AccessControlUpgradeable, PausableUpgradeable, ERC4626 {
  using FixedLib for FixedLib.Pool;
  using FixedLib for FixedLib.Position;

  /// @notice Tracks account's fixed deposit positions by maturity, account and position.
  mapping(uint256 => mapping(address => FixedLib.Position)) public fixedDepositPositions;
  /// @notice Tracks account's fixed borrow positions by maturity, account and position.
  mapping(uint256 => mapping(address => FixedLib.Position)) public fixedBorrowPositions;
  ...

for which generate_interface creates the following function signatures:

    function fixedDepositPositions(uint256,address) external returns (Position memory);
    function fixedBorrowPositions(uint256,address) external returns (Position memory);

This causes the compilation error below:

Error: Identifier not found or not unique.
--> DiffFuzzUpgrades.sol:257:71:
|
257 |     function fixedDepositPositions(uint256,address) external returns (Position memory);
|                                                                           ^^^^^^^^