2.x.x Template struct

[adam-fowler]

To make HBMustacheTemplate Sendable I have had to change the structure of things slightly. These are breaking changes.

• HBMustacheTemplate is now a struct
• HBMustacheLibrary is a now a struct
• Instead of setting the library inside the template when it is added to a library, we pass it into the HBMustacheContext when rendering the template. For partials to work the template needs to be rendered via HBMustacheLibrary.render.

I didn't think we'd need a v2.0 of hummingbird-mustache but it looks like we will

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:exclamation: No coverage uploaded for pull request base (2.x.x@52663ae). Click here to learn what that means.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 2.x.x #22 +/- ## ======================================== Coverage ? 91.27% ======================================== Files ? 16 Lines ? 1032 Branches ? 0 ======================================== Hits ? 942 Misses ? 90 Partials ? 0 ```

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