Implemented readelf-like functionality.

[n1073645]

The code in this PR implements readelf-like functionality into CyberChef. It utilises the stream library for most of it's processing and I have attempted to make it agnostic between 32-bit and 64-bit.

I have tested it on both 32-bit and 64-bit ELF files.

The tests are pretty thin on the ground for this one but I could create a small, totally invalid, ELF to use.

[n1073645]

The stream library has a readString functionality but it requires knowing the size of the string in advance which I do not know hence I have had to reimplement it.

[n1474335]

I think it would be reasonable to modify the Stream library's readString function to support reading until a null byte. It does currently stop at the next null byte if it occurs before the length limit. Perhaps the numBytes argument should default to -1 and if this is the value it just consumes bytes until a null is found. I don't believe this would break any existing uses of that function (though I'd recommend searching for all references to readString and confirming).

On the testing front, I'd put an ELF or two in the tests/samples directory and use some expectedMatch regexes to confirm you get expected fields. There are some provided under the MIT licence here: https://github.com/JonathanSalwan/binary-samples