Refactor Value/ValueType methods into built-in traits

[IFcoltransG]

Replaces a number of methods on Value and ValueType with names like get_foo_value and new_foo into a single get and new method generic over TryInto<foo>.
With the stylistic change from different methods to generics, comes auto-generated Value constructors from ValueType constructors rather than declaring them all manually, but the main reason is I think it's a bit cleaner to use Rust's built-in traits.