Add hasEvent and hasFunction methods to contract and interface

[farwayer]

Motivation:

Sometimes we need to check if the contract that was created in another part of the code contains the function or event we need. Now we have to call the getFunction/getEvent function and catch the exception.

This PR adds a simple check for the presence of functions and events in the interface. Note that these methods do not check that the contract actually supports them.

Handling of the has trap for the contract and filters has also been added. So the check can now be done, including using the in operator:

if ('transfer' in contract) {...}
if ('Transfer' in contract.filters) {...}

It is good behavior to override get and has proxy traps at the same time.

[ricmoo]

This is a great idea! I have a few other changes that are going out first, but will mark this for the next minor bump (after 6.2.0, which should go out later today).

[ricmoo]

Quick note; since the Contract is a meta-class, new methods cannot be added to it. But the rest looks good. I'm trying it out now. :)

[ricmoo]

Merged in v6.3.0.

Thanks for the suggestion! :)