Collect unset values in std::template and raise them all at once

The future module has a performance improvement for templates which have lots of unset values: it tries to resolve them all, catch all unset once and raise them all at once, such that the compiler doesn't re-evaluate the template every time one value is resolved, but only once they all are.

• https://code.inmanta.com/solutions/modules/future/-/blob/master/inmanta_plugins/future/std/monkeypatch.py?ref_type=heads#L117
• https://code.inmanta.com/solutions/modules/future/-/blob/master/inmanta_plugins/future/std/__init__.py?ref_type=heads#L117
• https://code.inmanta.com/solutions/modules/future/-/blob/master/inmanta_plugins/future/std/__init__.py?ref_type=heads#L415

This is done in another plugin (future::std::jinja) which has the same function. This plugin also supports references, which std::template doesn't, but this is out of scope for now.

Import the performance improvement, and make sure it is tested.

Other place where the improvement exists, which may (or may not) contain more test coverage: https://github.com/edvgui/inmanta-module-files/blob/6922a2e84aafc50659b1cadc601f77a958010756/tests/test_jinja.py#L241