attributes: permit #[instrument(follows_from = …)] (#2093)

This PR extends the `#[instrument]` attribute to accept an optional
`follows_from = …` argument that supplies any number of
`Span::follows_from` relationships to the generated `Span`.

## Motivation

This PR resolves #879.

## Solution

This PR largely follows the implementation strategy articulated by
@hawkw:
#879 (comment)

In that comment, @hawkw suggests taking one of two approaches:
1. each `follows_from` relationship is supplied with a distinct
   `follows_from` argument
2. the `follows_from` argument is provided once, and its value is a
   **list** of indirect causes

I take the second approach, since it is slightly more flexible: it
allows for the number of indirect causes to vary at runtime.

This addition is complemented by changes to `tracing-mock` to permit
making `follows_from` assertions for testing purposes.