Replace module-level dict by immutable data type

[McSinyx]

For instance, device names can be handled more elegantly using named tuples. However, as discussed in cython/cython#2552, NamedTuple (or any class requiring type annotation) is not yet supported. I am noting this here to not forget about it.

[McSinyx]

For device names specifically, neither'd be wise choice, since both are mutable data types. I can imagine palace is imported multiple times by downstream (which imports the same instance) and one of the downstream accidentally modify the data. That's be messed up.

[McSinyx]

With Cython extension type, one can use cdef readonly for attributes to make a class for these value.