Fix nested partial indenting

[cmrschwarz]

Currently, nested partials don't indent correctly.
This is due to two problems:

• The indent in RenderContextInner is not built up correctly
• After a stanalone partial, the next raw text is trimmed, so it will not be indented (no leading newline),
  causing it to be misindented.

This PR addresses both issues, causing a simple nested example (that was also added as a test case) to be indented correctly.

Example Input

{{#*inline "nested_partial"}}
<div>
    content
</div>
{{/inline}}

{{#*inline "partial"}}
<div>
    {{>nested_partial}}
</div>
{{/inline}}

<div>
    {{> partial }}
</div>

Output Before

<div>
    <div>
        <div>
        content
    </div>
</div>
</div>

Output After

<div>
    <div>
        <div>
            content
        </div>
    </div>
</div>

[coveralls]

coverage: 81.561% (+0.04%) from 81.519%
when pulling 18eeb6c on cmrschwarz:nested_partial_indenting
into 5dda501 on sunng87:master.

[sunng87]

LGTM. Thank you for fixing this long waited issue with my implementation.

The only concern is by adding a new variant to TemplateElement, we introduce a breaking change. The library is written from early days (2015) of rust when there is no #[non_exhaustive]. I should definitely add them to all public enums.