Refactor model state definition

[RomainFranceschini]

Use

state do 
    var foo : Type = value
    var bar : Type { lazy_value }
end

instead of

state_var foo : Type = value
state_var bar : Type { lazy_value }

Much less macro code used, state pattern now enforced.

[RomainFranceschini]

Travis CI failed (Crystal 0.34, LLVM 8.0.0) because both atomic_spec.cr and state_spec.cr declares two private classes with the same names. A type cast error happens only with LLVM 8.
Changing the names of the classes in one file seems to resolve the issue 🤷‍♂️