Support const fn implementation

[ZacJW]

Currently associated-derive just derives an implementation for Associated, but due to a language limitation trait methods cannot be const fn, which means the user cannot use .get_associated() to initialise a const.

One proposed solution is to optionally support generating a non-trait method on the enum which can then be const fn. In order to do so, the user should be able to supply a name for it to avoid name collision, but maybe default to get_associated. The macro will have to check that only #[assoc_const] variant attributes were used as #[assoc] wouldn't always work as it allows non-const expressions.

Issue discovered by a kind Rustacean on r/rust