Improved function bytes extraction to JSON file with masks

[valbucci]

Radare2 v6.0.0 introduced a new command p8fmj. This provides not only a function's raw bytes but some additional information as follows:

{
    "type": "function",
    "linear": true,
    "addr": 5368720672,
    "size": 6,
    "data": "ff254a970400",
    "mask": "7f0c00000000"
}

This PR proposes a new implementation of the bytes extraction mode, which extracts all function's byte data with p8fmj @@f and stores the dictionaries into an array which is then serialised and stored inside a single JSON file, instead of a folder.

The functions write_to_bin and get_bytes are therefore no longer used. They are retained in case we intend to implement a different extraction mode for retro-compatibility.

[valbucci]

I accidentally tried to merge this PR into main. Fixing it now.