Fill out more formatting functionality

[KodrAus]

Current PR:

This PR adds more functionality to our sval_fmt::TokenWrite trait so that it gives more fidelity for customizing formatting. There are some examples in the tests that may make their way into first-class adapters at some point that do compact formatting and indented formatting.

In the end I wasn't really satisfied with how tagging for text fragments came out, so I reverted it and just kept the improvements to TokenWrite and testing that came along with it.

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Original PR:

This PR implements two new methods for working with text:

• tagged_text_fragment(tag: &Tag, fragment: &'sval str)
• tagged_text_fragment_computed(tag: &Tag, fragment: &str)

These make it possible to use tags for tokenizing text without changing the representation. Since each fragment can only have a single tag it's a fairly limited system, but capable enough to support things like syntax highlighting or optional formatting.

[KodrAus]

I've also made the Computed stream adapter public, since it's a useful way to get yourself a Stream<'b> from any Stream<'a> to avoid duplicating logic when you may or may not have data satisfying 'a.