cmd: implement abidump

[holiman]

This PR implements abidump, a utility to parse calldata against the fourbyte directory database.

[user@work abidump]$ ./abidump 
Error: one argument needed
Usage: ./abidump <hexdata>

Parses the given ABI data and tries to interpret it from the fourbyte database.

[user@work abidump]$ ./abidump a9
WARNING: Transaction data is not valid ABI (missing the 4 byte call prefix)

[user@work abidump]$ ./abidump a9059cbb
WARNING: Transaction contains data, but provided ABI signature could not be verified: signature "function transfer(address , uint256 ) returns()" matches, but arguments mismatch: abi: cannot marshal in to go type: length insufficient 0 require 32

[user@work abidump]$ ./abidump a9059cbb000000000000000000000000ea0e2dc7d65a50e77fc7e84bff3fd2a9e781ff5c0000000000000000000000000000000000000000000000015af1d78b58c40000
Info: transfer(address: 0xEA0e2Dc7d65A50E77FC7E84bff3FD2A9E781ff5c,uint256: 25000000000000000000)

[holiman]

@karalabe you requested this once upon a time, over discord... Want it?

[holiman]

Rebased, let's see what travis thinks of it now

[holiman]

[gballet]

it's 2020 now

[holiman]

but it wasn't when I wrote the code

[gballet]

you didn't specify a license when you wrote the code 😁

[gballet]

LGTM