allow from_string to be specified via metadata

[minrk]

avoids need to define custom Trait subclasses just to define how strings are parsed, e.g.

class C(Configurable):
    path = List().tag(config=True, from_string=os.pathsep.split)

[rmorshea]

Why not look this up in the metadata dict rather than assign to an attr and find it there? Or if from_string can be a method on the class, use the metadata as a fallback. Seems like it would confusing or hard to debug if from_string in metadata were allowed to somewhat dynamically overwrite a method.

[rmorshea]

Nevermind, given the point is to overwrite methods this makes sense.

[minrk]

Yeah, it's the TraitType subclass priority I was thinking of. The alternative was to implement _from_string private method that does the lookup, but I thought that might get confusing.

[minrk]

I was about to merge this when I realized the argument should probably be a Bunch with access to the trait instance itself like we do with all other hooks, to grant access to other tags, default values, etc.

[minrk]

Hm, but that makes it not have the same signature as the method. Maybe I should make the signature (trait, string) so it's the same as the method? I'm not sure.

[rmorshea]

I think making it (trait, string) would be best. Simpler and more inline with the signature of the method.