Use strict Vector in Value.

`KeyMap` (object) is strict values. I don't see a reason why array values couldn't as well.

This will be a major breaking change though, comparable to `aeson-2.0.0.0` breakage.

Start by tracing the Value representation and the existing strictness of KeyMap, then examine how array values use Vector. Assess the compatibility impact of making Vector strict, including the breaking-change concern compared with aeson-2.0.0.0, and define the expected behavior before implementation.

Written by the indexing model from the issue text.