Merge: introduce Sequence::prepend

How could people live without having a working prepend?

Pull-Request: #675
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc lib/standard/collection/array.nit
Simple merge

diff --cc tests/sav/test_new_native_alt1.res
index b739bff,855a24d..0d3a41d
--- 1/tests/sav/test_new_native_alt1.res
--- 2/tests/sav/test_new_native_alt1.res
+++ b/tests/sav/test_new_native_alt1.res
@@@ -1,4 -1,4 +1,4 @@@
- Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:740)
 -Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:759)
++Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:753)
  NativeString
  N
  Nit