Fix loading large non file-visiting buffers + simplification of the implementation

[abougouffa]

This PR fixes #5 and simplifies the implementation in these manners:

1. There is no need to advice find-file-noselect, the file will be opened in all cases, and if we want to check for the /file size/ or for /too long lines/, we can do it inside the set-auto-mode-0 advice function (using the current buffer and the (buffer-file-name)).
2. The current implementation can cause some extra overhead by opening the file to detect too long lines (why we open the file temporarily while we can check for long lines inside set-auto-mode-0's advice?)
3. So, the guard-lf--line-too-long-p function has been modified to work on a buffer instead of a file (to avoid opening the file temporarily).

Edit

I think, by using the buffer-size to calculate the current buffer size, we don't even need to get the file size from the file system's attributes. At the set-auto-mode-0 level, the file would be already inserted in the current buffer, so we can relay on (buffer-size)!

[jcs090218]

Make sense! Thank you!

[abougouffa]

Thank you for your responsiveness! @jcs090218