Character validation/handling for URIs

[carueda]

This is about a general review/handling of user-given strings that are either supposed to be complete IRIs, or used to compose IRIs (e.g., "local names").

In code adjustment for #33 we are currently restricting some characters that are in general considered unsafe (so this is OK), but also some that are not yet properly handled. For example, parens should be OK in a URI's local name, but are avoided at the moment.