Reduce allocations by using #each_with_object

[ashmaroli]

Summary

The loop within config.reduce({}) { |hsh, (k, v)| hsh.merge(k.to_s => v) } is going to run for each
key-value pair in config and on each iteration, hsh.merge is going to allocate a new Hash object.

Therefore, if a config file has 20 top-level key-value pairs, this method would've allocated at least 20 Hash objects (of increasing size)..
This can be bad for sites that have numerous top-level config keys (e.g. forks of minimal-mistakes theme repo)

[parkr]

Functional programming can be fun, but isn’t always efficient in this wild world of Ruby we’re living in. 😄 Thanks for fixing this up!

[ashmaroli]

@parkr Sorry, I found one more instance of Hash[array.map { |k| [k, foo(k)] }] after you approved the PR.

[ashmaroli]

The gains aren't much, but probably non-trivial for much larger sites:

Profiler Summary

--- master branch https://travis-ci.org/jekyll/jekyll/jobs/561428064
+++ PR branch     https://travis-ci.org/jekyll/jekyll/jobs/562630268

- Total allocated: 479.11 MB (4483645 objects)
- Total retained:  18.54 MB (96252 objects)
+ Total allocated: 479.06 MB (4482911 objects)
+ Total retained:  18.54 MB (96251 objects)

[DirtyF]

@jekyllbot: merge +dev