toml_value_to_labelset converter should cast list elements to str

[NickleDave]

either by doing

labelset = set([str(element) for element in toml_value)

or

labelset = set(''.join(toml_value))

trying to figure out if either can cause / avoid errors.
Option one is more readable I think. Option two joins everything into one string before passing it to set which will then split it up into individual characters again.

Probably there should be a validator on labelset that makes sure it's actually a set?

toml arrays have to be "homogenous" (all elements same type) right now AFAI can tell
toml-lang/toml#553