ParseWrapper and TokenStreamWrapper can't work with internal buffering

[sunshowers]

ParseWrapper and TokenStreamWrapper receive their tokens through a side channel that only works while our deserializer is in charge. This doesn't really work with scenarios in which serde performs internal buffering -- it calls deserialize_any to buffer the value as a Content first, and only later hands that Content to the wrapper's Deserialize impl. Our deserialize_bytes is never called, so the side channel never fires and the wrapper sees a plain string or number.

In general, this can't be fixed. Spans can't be serialized, so they can't be roundtripped through Content. (We could try doing some kind of after-the-kind fixup but that seems dubious.) The alternative is to gracefully degrade by using Span::call_site(), but I'd rather put the burden on macro authors to fix their types instead.

Currently, any attempt to use these wrappers in such a scenario results in the somewhat unhelpful error message:

invalid type: string "foo", expected TokenStream

Improve this error message to indicate that the macro needs to be fixed, and document the limitation on both wrappers. This error reaches the user as a spanned diagnostic (rather than a panic) due to the span attribution in the *Access impls done earlier in this series.

Note that this error message isn't seen with #[serde(untagged)] because it swallows each variant's error. But the underlying principle remains the same.