subscriber: use ANSI formatting in field formatters

[hawkw]

Depends on #1696

Motivation

PR #1696 adds a new API for propagating whether or not ANSI formatting
escape codes are enabled to the event formatter and field formatter via
the Writer type. This means that it's now quite easy to make field
formatting also include ANSI formatting escape codes when ANSI
formatting is enabled. Currently, tracing-subscriber's default field
formatter does not use ANSI formatting --- ANSI escape codes are
currently only used for parts of the event log line controlled by the
event formatter, not within fields. Adding ANSI colors and formatting in
the fields could make the formatted output nicer looking and easier to
read.

Solution

This branch adds support for ANSI formatting escapes in the default
field formatter, when ANSI formatting is enabled. Now, field names will
be italicized, and the = delimiter is dimmed. This should make it a
little easier to visually scan the field lists, since the names and
values are more clearly separated and should be easier to distinguish.

Additionally, I cleaned up the code for conditionally using ANSI
formatting significantly. Now, the Writer type has (crate-private)
methods for returning Styles for bold, dimmed, and italicized text,
when ANSI escapes are enabled, or no-op Styles when they are not. This
is reused in all the formatter implementations, so it makes sense to
have it in one place. I also added dimmed formatting of delimiters in a
couple other places in the default event format, which I thought
improved readability a bit by bringing the actual text to the
forefront.

Example of the default format with ANSI escapes enabled: