Replace R-assignment by single-line pattern matching.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

test/t/syntax.rb

@@ -345,13 +345,9 @@ assert('splat object in assignment') do
   assert_equal [2], (a = *o)
 end
 
-assert('right-ward assignment') do
+assert('one-line pattern match') do
   1 => a
   assert_equal(1, a)
-  13.divmod(5) => a,b
-  assert_equal([2,3], [a, b])
-  13.divmod(5) => a,b => c, d
-  assert_equal([2,3,2,3], [a, b, c, d])
 end
 
 assert('splat object in case statement') do