Splits MakeValueWriter into two traits

[zslayton]

This patch contains no logic changes, only a refactoring of the MakeValueWriter trait.

Instances of ValueWriter are single-use by design, as this allows the crate to
statically guarantee that an application cannot (for example) try to encode multiple
values following an annotations sequence or inside a struct field.

Prior to this PR, the MakeValueWriter trait was public for testing purposes.
This allowed applications to refer to a container's associated ValueWriter family of types,
which was needed for custom writer methods leveraging generics.
For example, a list writer at the top level of a stream might be written as:

let mut list_writer: 
    <
        <Writer<v1_1::Binary, File> as MakeValueWriter>
            ::ValueWriter<'_> as ValueWriter
    >
    ::ListWriter 
= // ...;

(This is admittedly very ugly, but alas there is no way to offer a nice shortcut for this case.)

While exposing MakeValueWriter allowed applications to concretely reference associated type paths,
it also exposed the make_value_writer method. make_value_writer needs to be kept private to
prevent users from circumventing ValueWriter's statically enforced single-use restriction,
creating and then using multiple value writers in situations where that would produce illegal data.

This PR splits MakeValueWriter into two traits:

1. ContextWriter, which houses the associated types
   and
2. MakeValueWriter, which houses the make_value_writer method and, by extending ContextWriter, has access to the necessary associated types

ContextWriter is public/re-exported, allowing applications to refer to associated types as needed.
MakeValueWriter is now pub(crate) and is no longer re-exported at the top level.

The example variable declaration above would now be written:

let mut list_writer: 
    <
        <Writer<v1_1::Binary, File> as ContextWriter>
            ::NestedValueWriter<'_> as ValueWriter
    >
    ::ListWriter
= // ...;

This PR supersedes #864 and fixes #863.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.