Make procedural documentation compile

[cheme]

Currently some procedural macro documentation code snippets are defined as nocompile. For instance decl_storage documentation example.
Making the example build will allow to avoid many regression.
At the time, the task is not easy due to the way hidden_include works (documentation code snippet is seen as a part of the crate so no include is generated, but that is not the way rust doc works).
Moving doc does not really help to.
This could probably be solve in future rust version, depending on proc_macro evolution. It may also be solved by making the hidden_module tools compatible with documentation (not sure it is worth it).

Issue created for #1437.