Add benchmark for sequential assignment.

Calculating -------------------------------------
 parallel assignment    126425 i/100ms
multi-line assignment
                        136492 i/100ms
-------------------------------------------------
 parallel assignment  6382145.2 (±10.1%) i/s -   31606250 in   5.006184s
multi-line assignment
                      9281041.8 (±11.7%) i/s -   45724820 in   5.001599s

benchmark/sequential-assignment

@@ -0,0 +1,11 @@
+require 'benchmark/ips'
+
+Benchmark.ips do |x|
+  x.report('parallel assignment') do
+    a, b = 1, 2
+  end
+  x.report('multi-line assignment') do
+    a = 1
+    b = 2
+  end
+end