Refactor string parameters to ReadOnlySpans

Where appropriate, ReadOnlySpan<char> should be preferred as method parameters over default string type.

Ideally, these methods should also have string, char[], and Span<char> overloads that delegate down to the ReadOnlySpan<char> version.

Other overloads are possible when dealing with strings, though none natively convert to spans, so they can't be directly delegated down to the ReadOnlySpan<char> version.

First their are the collections:

• IList<char> provides a count and element access.
• ICollection<char> provides count, but no direct element access.
• IEnumerable<char> doesn't provide a direct count or element access.

Then we have the wrapper objects:

• StringBuilder is seekable, can read/insert/append/removal, not line- or indent-aware.
• TextWriter isn't seekable, only append, not line- or indent-aware.
• IndentedTextWriter isn't seekable, only append, is indent-aware, and not line-aware.
• TextReader isn't seekable, only read, is line-aware, not indent-aware.