subscriber: replace dyn Write with a Writer type

[hawkw]

Motivation

Currently, the FormatEvent and FormatFields traits in
tracing-subscriber are passed a &mut dyn fmt::Write trait object to
write formatted representations of events and fields to. This is fine,
but it doesn't give us the ability to easily provide additional
configuration to the formatter, such as whether ANSI color codes are
supported.

Issue #1651 describes some approaches for adding a way to expose the
ANSI color code configuration to custom formatters. In particular, the
proposed solution involves wrapping the fmt::Write trait object in an
opaque struct, which can then implement additional methods for exposing
information like "are ANSI colors enabled" to the formatter. Since this
changes the signature of the FormatEvent::format_event and
FormatFields::format_fields methods, it's a breaking change.
Therefore, we need to make this change now if we want to get the API
change in for tracing-subscriber 0.3.

Solution

This branch adds a Writer struct that wraps the &mut dyn fmt::Write
trait object, and changes the various method signatures as appropriate.
It does not actually implement the change related to ANSI color
formatting. Once we change these methods' signatures to accept a
Writer struct, we can add as many methods to that struct as we like
without making additional breaking API changes. Therefore, this branch
just makes the breaking change that's necessary to get in before v0.3
is released.

Propagating the ANSI color configuration can be implemented in a future branch.

[yaahc]

looks good insofar as tracing-error is concerned, once the build failures are resolved.