refactor: turn off pub access of chain in HelperTemplate

[sunng87]

Because chain is not used during rendering process, here I turned off pub access of this field for better compatibility.

[coveralls]

coverage: 81.651%. remained the same
when pulling c1a4aef on fix/turn-off-chain-field-of-helper-template
into d8d9a78 on master.

[epage]

Wasn't this a breaking change?

Before: https://docs.rs/handlebars/5.1.0/handlebars/template/struct.HelperTemplate.html
After: https://docs.rs/handlebars/latest/handlebars/template/struct.HelperTemplate.html

Both removes access to a field and makes the struct no longer directly initializable. If the latter is an expected property, this should have #[non_exhaustive] (but that too is a breaking change)

[sunng87]

I see. I didn't realized initialization issue with introducing a private field. Will revert this and yank 5.1.1 for new solution.

[epage]

This isn't just about initialization but also that a release already exists with pub chain

[sunng87]

This field is introduced in 5.1.0 and not used in any API. The impact should be minimised.