Reword changelog

[ci skip]
Use stable rust for test suite
Perform important teardown tasks first

Delete files and kill processes before doing anything else.
Simplify code
Use generalized push instead of setq/cons
Define backwards compat functions as inline

Just like subr-x does
Use string-empty-p
Simplify argument substitution

flycheck-substitute-argument always returns a list now, which reduces
the -flatten/-keep dance to a simple append/mapcar.
Simplify flycheck-prepend-with-option

Wrap a list around a string result from prepend-fn, and replace -flatten
with a simply apply/append.
Improve docstring

[ci skip]
Replace -keep with delq nil/mapcar

Simplifies a bunch of expression, and removes another redundant dash.el
function
Replace !cons with standard push
Replace -first with while loops
Replace -group-by with a straight-forward loop
Add info pattern to rust checker
Add Scalastyle checker

Utilizes Scalastyle from http://www.scalastyle.org
[Fix #425] Merge branch 'scalastyle'

* scalastyle:
  Add Scalastyle checker
Update changelog and credits [#425]
Mention the variables in scalastyle docstring
Fix GHC path on Travis CI