Improve metadata handling #59

Following the trailofbits/manticore#909 discussion:

We need to improve

To handle newer versions of the metadata encoding, including:

• 0.5.15

0xa2
0x65 'b' 'z' 'z' 'r' '1' 0x58 0x20 <32 bytes swarm hash>
0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding>
0x00 0x32

• 0.6.1

0xa2
0x64 'i' 'p' 'f' 's' 0x58 0x22 <34 bytes IPFS hash>
0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding>
0x00 0x32

We could also handle the parsing of the metatadata, like doing the key-values extraction.

Additionally, we should add a public function allowing external tools to access these functionalities. This would be useful for, at least, Manticore and evm-cfg-builder.