Explicitly declare decl_storage! getters as functions

[rphmeier]

This is a backwards-compatible (test included) convenience improvement for decl_storage!.

Instead of declaring a field as

MyField get(getter_name): ...;

we can declare as

MyField get(fn getter_name): ...;

This makes it clear that a function is being declared, and also means that searches for fn getter_name don't come up empty when reading code and looking for definitions. I also ported over all our SRML modules to follow this pattern.

[bkchr]

Could you please update the docs of the macro for this change?

[rphmeier]

could you tell me which docs you mean? i did another pass, but I had already gotten through srml-support/procedural. There aren't any other obvious docs.

[kianenigma]

Good stuff! The plan is to stay backward compatible or discard get(something) at some point?