Ensure a clear definition of the base URI

[wismill]

It is not always clear what the "base URI" means in various places of this library (see this comment.

Therefore we should be more precise in naming and documentation to ensure consistency.

References:

• RFC3986, section 5.1
• https://www.w3.org/TR/turtle/#sec-iri-references

[robstewart57]

@wismill your xmlf monad transformer code has been added into this repo, including IRIs and IRI validation/resolution. Could your IRIs now be used more widely throughout this library?