Support iterating over maps/objects

[jamespfennell]

Thank you for this very nice project. I love the simple and low-dependency philosophy.

Somewhat not in that vein: what would you think about supporting iteration over maps/objects? Currently iteration is restricted to arrays. Following the standard syntax in other templating languages, it could look something like this:

{{ for key, value in guests }}
The key is {key} and the value is {value}
{{ endfor }}

I poked around the code to see what the implementation would be like, and I think it would be straightforward (I would do it):

• We would add a new instruction similar to PushIterationContext. Perhaps PushObjectIterationContext(Path<'template>, &'template str, &'template str) where the enum payload is (path to the map, key variable name, value variable name).

• In the compiler, the parse_for function would be changed to split the key name on ,, and then construct either a array iteration spec (no commas), an object iteration spec (1 comma) or an error (2 or more commas).

• The new instruction would have to be implemented in template.rs. This would follow array iteration, probably with a new ContextElement.