Optimization: Replace control-flow-by-exception with magic testing and incremental reads.

[woodruffw]

Instead of attempting to open a file with MachOFile.newand catching
a FatBinaryError to attempt FatFile.new, read a few bytes and do some
magic/sanity tests. Once sanity is established, instantiate either a
MachOFile or a FatFile.

Tasks:

• Add attr_accessor filename
• Open and check the file directly in MachO.open
• Unit tests

Reference: #7

[woodruffw]

@UniqMartin Mind doing a quick lookover of this?

[UniqMartin]

Mind doing a quick lookover of this?

Looks good to me! (And sorry for being slow to respond!)

Are you intending to make changes to FatFile and MachOFile too, possibly even changing them in a way such that they don't have to read the entire file to do their work? I admit that shouldn't be such a pressing issue now that the MachO.open method is much more efficient (after this PR is merged), just wondering about your plans. 😃

[woodruffw]

Looks good to me! (And sorry for being slow to respond!)

Thank you (and no worries)!

Are you intending to make changes to FatFile and MachOFile too, possibly even changing them in a way such that they don't have to read the entire file to do their work? I admit that shouldn't be such a pressing issue now that the MachO.open method is much more efficient (after this PR is merged), just wondering about your plans.

Yeah, this would be ideal. I probably won't get around to it in this PR, but it would ultimately be much nicer to spread these performance improvements around if and when users choose to create MachOFile and FatFile instances directly.

I'll merge this tonight!