WIP: Filecopy sha256

[mster]

REF: #21 #19

Opening a work in progress PR for tracking progress on this work.

@nick-nuti has implemented SHA256 hashing! 🧙

CLOSES: #21 #19 #22

[mster]

Changes:

• Breaks out SHA2556 functionality to crypto.cpp/.hpp
• Timestamp is now in unix epoch form
• BackupParams now includes a ts property, which is the UNIX epoch timestamp of when the program executed.
• Remove change_directory usage from buildDigest
• Remove commented code
• Breaks filecopy (passed path is malformed)

TODOs:

• filecopy is broken. We need to look into using absolute paths instead of setting the current directory. Currently the path is malformed. An extra / is being added somewhere. I suspect it's at https://github.com/nick-nuti/Backup-Program/blob/filecopy-sha256/src/filecopy.cpp#L101
• I'm strongly for using absolute paths. We will see more of the benefits down the line. I'm not sure saving a few bytes is worth sacrificing code clarity/testability. We should remove all uses of current_path.
• All cryptographic operations should be broken out. If we need to write to a temporary file for calculating hashes, this shouldn't be included in the tree building functions. Keeping a separation of concerns is crucial. https://github.com/nick-nuti/Backup-Program/blob/filecopy-sha256/src/tree.cpp#L119-L130
• Assure variable names are sane. For readability we need to make sure declarations make sense.

• 27

@nick-nuti

Closes #22

[mster]

You can see the current changes here: https://github.com/nick-nuti/Backup-Program/tree/filecopy-sha256

OR

The diff here: https://github.com/nick-nuti/Backup-Program/pull/24/files