v0.16 release planning

[GuillaumeGomez]

• It should error when a same block is defined more than once #668
• Support for jinja's block assignments #511
• Add syntax for argument type declaration for macros #514
• Support executing caller() method in variable declaration #506
• Switch to std::path::absolute to avoid symlinking template paths #720
• Limit recursions for Ty generics #723
• Add support for tuple, array and slice in type generics #721
• Turn duplicated block warning into an error #711
• Add support for let blocks #697
• Implement compound assignments (e.g. {% mut a += 1 %}) #685