Cop idea: prefer `a.chain(b)` over `a + b` when iterating

[vlad-pisanov]

.chain can link multiple Enumerables into a single efficient iterator.

# bad
(a + b).map { |x| 42 + x }

# good
a.chain(b).map { |x| 42 + x }

Unlike +, it doesn't allocate an intermediate result:

require 'memory_profiler'

a = (1..1000).to_a
b = (1..1000).to_a

MemoryProfiler.report {    (a + b).map {} }.pretty_print
MemoryProfiler.report { a.chain(b).map {} }.pretty_print

with +:

allocated memory by class
-----------------------------------
     32080  Array

with chain:

allocated memory by class
-----------------------------------
     17784  Array
       160  Proc
        56  Enumerator::Chain