Merge: Simplify management of primitive types

Add direct methods to access primitive types

Simplify and improve the generation of primitive values in the compiler.

More (and improved) `*_instance` methods are now available in AbstractCompilerVisitor.
One of the point is the simplification of the generated C so that less local variables are generated, maybe this will also help the C compiler to work faster.

Pull-Request: #1213
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/compiler/abstract_compiler.nit
Simple merge

diff --cc src/compiler/global_compiler.nit
Simple merge

diff --cc src/compiler/separate_compiler.nit
Simple merge

diff --cc src/compiler/separate_erasure_compiler.nit
Simple merge