Split file contents in the lines only once

[popzxc]

It is inefficient that ModifiedFile content is split into lines by each lint separately, since the expensive operation is repeated many times while it can be done only once.

One more thing is that List may not be the most efficient structure to store lines, since some lints (e.g. NoNewlineAtFileEnd) don't need to process all the lines, but instead need one concrete line (e.g. last). Thus Vector may be a better choice.

As a conclusion, the proposed solution is to add a field like mfLines to the ModifiedFile type, and perform splitting once in Hint.hs (but the raw content should be available as well, no need to remove mfContent).

For details see a discussion at #156.