Version 0.1.6: Optimizations and improvements

Some language improvements:

• it used to be impossible to declare global variables in modules and then refer to them from functions. This now works.
• JSLT now forbids object literals that contain the same key twice.
• JSLT now depends on Jackson 2.9.8.

One function has been added:

• mod(a, b) to compute a modulo b. (Thank you to @ecerulm for work on this!)

This version has been optimized in several ways:

• variables that are used only once, or that have constant values, are now optimized away completely
• operations involving two literals are now performed at compile-time,
• the internal representation of variables is now much faster, and
• if b in contains(a, b) is a large array literal, the optimizer now replaces this with a hashset lookup, instead of doing a linear search.