Don't let ack take up all memory

[hoelzro]

In some (fairly) recent changes to ack since 2.14, I created an optimization by reading in a file in its entirely and calling the regex engine on that giant blob instead of applying the regex line by line. This is great, as it results in a giant speed boost. What's not so great is when ack mistakenly starts searching a huge file that consumes all memory on the machine. So there are a few ways we can get around this:

• Before ack starts searching a file, check its size. If it's over some preset limit, fallback to line-by-line searching.
• Read a file in fixed size chunks (perhaps the same chunk size as stdio uses) and scan those.
• Use mmap (this could be the default, falling back to one of the other two if mmap isn't available)

[hoelzro]

For the mmap route, Sys::Mmap and File::Map seem to be popular choices.

[petdance]

There will probably be no more releases of ack 2. If this is still a relevant idea, please submit it to the ack3 repo at https://github.com/beyondgrep/ack3.