Optimize hashrocket alignment for speed

The way the function is currently written, it is processing the same
lines over and over again, which can make things very slow if we're
processing an entire file that is big.

In order to optimize what the function is doing, I made
puppet#align#AlignHashrockets report back what line indexes it has
worked on and then we accumulate the line indexes already seen so that
we can skip over them when we encounter those lines again in our loop.

As an example of a slow-to-process file, I've found that in
https://github.com/puppetlabs/puppetlabs-apache the file
manifests/vhosts.pp, which is 2967 lines long was particularly slow to
process with the current code.

If I open that file with nvim/vim, then go to the first line and
reformat the whole file (gggqG), it takes the plugin:

nvim 0.9.5: elapsed time:   48.235387
vim 9.1: elapsed time:   76.116376

With the optimization in place, the plugin takes:

nvim 0.9.5: elapsed time:    0.101734
vim 9.1: elapsed time:    0.12892

so about two orders of magnitude faster.

The above timings were taken with a CPU 12th Gen Intel(R) Core(TM)
i5-1240P and 32 Gb of RAM, so your measurements may vary a bit.