rename Output to XmlString, use that everywhere

In practice I found that I would often miss out on stripping ANSI escape codes
from various bits. Rename `Output` to `XmlString` and use it as a newtype.

Also implement a bunch of conveniences to allow `XmlString` instances to behave
like `str`.

Author: sunshowers

Cargo.lock

@@ -23,3 +23,5 @@ uuid = "1.10.0"
 [dev-dependencies]
 goldenfile = "1.7.1"
 owo-colors = "4.0.0"
+proptest = "1.5.0"
+test-strategy = "0.4.0"