std: add call_with_default and std::evm::erc20 safe-transfer helpers

[cburgdorf]

Typed external calls strictly decode returndata, so a -> bool message reverts when a token (USDT-style) executes but returns no data. Add two opt-in layers; the strict call<M> path is untouched:

• Call::call_with_default<M> (+ Address::call_with_default): identical to call<M> except that a successful call with EMPTY returndata yields a caller-supplied default instead of a decode revert. Reverts still bubble verbatim and non-empty returndata is still decoded strictly. Caveat (as in Vyper's default_return_value): a call to a codeless address yields the default.

• std::evm::erc20 with safe_transfer / safe_transfer_from / safe_approve (OpenZeppelin SafeERC20 semantics): tolerates empty returndata, reverts on a false return, and closes the no-code hole with an extcodesize check performed only on the empty-returndata path.

Fixture tests cover the full mock matrix for both layers: compliant, reverting (revert data asserted verbatim), no-return, false-returning, malformed (<32 bytes) returndata, and no-code targets, plus a non-bool return type exercising both the empty and non-empty paths.

[cburgdorf]

I'm not quite sure if we really want the erc20 stuff in the std lib or if that should be in an external ingot. But I think the call_with_default is a good API to deal with the different ways that a call might return.

[sbillig]

I'm not quite sure if we really want the erc20 stuff in the std lib or if that should be in an external ingot

For now I think std should have rich support for all common interfaces. We can optionally kick it out to an external ingot later.

[cburgdorf]

@sbillig sounds good. Then, this should be ready for human review.