Minijinja

- Restructured the value type internally to be simpler and not use unsafe at the
cost of slightly larger memory footprint. (30)
- Added `debug` support. If the debug mode is enabled, errors now carry a lot of
useful debug information and an alternative representation when formatted into
strings that show the context of the template where it went all wrong. (31)
- Added automatic string interning of object/map keys in values. This feature can
be disabled by removing the default `key_interning` feature. (35)
- Removed deprecated `Single` type.

- Remove one trailing newline to be consistent with Jinja2.
- Resolved a bug where borrowed keys on dynamic objects could not be looked up. (29)

- Restored unconditional compatibility with 1.42.

- Turned on all features for the docs on docs.rs

- Added `context!` and deprecate `Single`.
- Correctly report template file names in errors.
- Added the `source` method on templates.
- Introduced `State` type and changed parameter to functions from
`&Environment` to `&State`.
- Added `debug` global.
- Added `tojson` filter.
- Added `urlencode` filter.

- Made the `source` method on error be bound to `Send` and `Sync`.