Provide parse functionality to aid in serialization

**Description**
This is a feature request.

**Steps to reproduce the issue:**
1. Use `quad.ToString(quad.IRI("val"))` to create `""`
2. Attempt to parse it using methods provided in `quad` package

**Received results:**
`quad.MakeRaw` which has been marked as deprecated, will produce correct results
`quad.MakeIRI` suggested in deprecation notice will create an IRI of IRI `<>` which is invalid.

**Expected results:**
I expected to see a pair of methods clearly noted to be used from Marshalling and Unmarshaling `quad.Value`.

Start in the quad package by tracing ToString, MakeRaw, and MakeIRI for quad.Value serialization and parsing. Determine how Marshalling and Unmarshaling are currently represented, then verify that the intended paired functionality handles the shown IRI example without producing nested IRIs.

Written by the indexing model from the issue text.