Improve meson build.

-Add all required packages as wrap dependencies
-Move source lsitnings to separate files
-Proper dependencies declarations
-Set global D compiler arguments
-Add few asserts
-Add test exe
-Other small improvements

Tested with dmd with betterC, demos executable has to be run from 'demos' directory to properly load assets.

subprojects/bindbc-loader.wrap

@@ -0,0 +1,7 @@
+[wrap-git]
+url = https://github.com/BindBC/bindbc-loader.git
+revision = 9a51af991acce3c67e51695c07bf3fa6419ef938
+patch_directory = bindbc-loader
+
+[provide]
+dependency_names = bindbc-loader