VariableList should implement TryFrom<Vec> instead of truncating

[benluelo]

I was quite surprised to see that VariableList implements From<Vec> - this should be a TryFrom implementation instead. As per the docs on From:

Note: This trait must not fail. The From trait is intended for perfect conversions. If the conversion can fail or is not perfect, use TryFrom.

[michaelsproul]

I agree this would be a nice change to make. We can add it to ssz_types first in a new v0.6 release, then integrate it into Lighthouse at a later date.

Lighthouse is also moving towards milhouse types instead of the ones in this crate. Our tree-states branch already removes usage of From<Vec<_>> because milhouse doesn't support it: https://github.com/sigp/milhouse/blob/e30719aaf1e4b46e3ff1962f7370965e68fa548b/src/list.rs#L45-L47.

[michaelsproul]

Sorry, missed this for 0.6 and 0.7. Will try to get it in for 0.8