Support structs and enums in ABI

[chriseth]

Implement the following specification: ethereum/EIPs#50

[chriseth]

This most likely also fixes #980

[chriseth]

Just a note to myself: The encoder for tuples / structs should work as follows:

// nextfree is the place where we can allocate memory,
// all pointers are absolute memory positions
encodeTupleToMemory_t1t2t3...tn(from, to, nextfree) -> (next, nextfree_)
{
    encode values from "from" to "to", passing and updating "nextfree"
    store offsets relative to "to"
    return (from + 'head size', updated nextfree)
}