centralize validValueOrId check

[mathdroid]

          just an idea, not needed for this version: this logic could be a function since we reuse it everywhere, it's critical, and when we want to support burning it will need to be changed in many places.

Originally posted by @caldereth in #17 (comment)

[aruns05]

@mathdroid is this being worked upon ?

[mathdroid]

it should already be in the current impl. closing after confirmation @0xacme @caldereth

[gaavar]

I think that the _isValidTokenId implements this and is present in the main branch.

[aruns05]

Is there any other good first issue,interested in contributing