Warn if built-in globals are shadowed

[codywatts]

"Built-in" functions such as sha3 and selfdestruct can be hidden by functions and events with the same name. This can alter the functionality of contracts in non-obvious ways.

Consider the following contract:

pragma solidity ^0.4.2;

contract FakingItsOwnDeath {
    function selfdestruct(address x) internal constant {}
}

contract SelfDestructive is FakingItsOwnDeath {
    function die() public {
        selfdestruct(address(0x0));
    }
}

At a glance, SelfDestructive.die appears to destroy the contract. But in fact, it's actually calling FakingItsOwnDeath.selfdestruct rather than the built-in version of selfdestruct and so the contract doesn't die.

It is also to possible to create functions/events called this and now.

(Tested using version 0.4.2+commit.af6afb04.mod.Emscripten.clang of the online Solidity compiler.)

[chriseth]

This is all intended but should probably result in a warning if built-ins are overwritten, yes.

[chriseth]

This is also very important for revert()