usd: Simplify CrateFile::_ValueHandler type hierarchy using C++17's 'if

constexpr()'.  This type hierarchy existed to let us simultaneously share
common code among all the kinds of value types (inlined-vs-not,
array-supporting-vs-not) while not instantiating code that wouldn't be valid
for one type or another.

Happily, 'if constexpr()' will totally discard the appropriate 'if' or 'else'
branch, giving us a cleaner way to achieve these goals.  This change collapses
the old type hierarchy:

_ValueHandlerBase <- _ScalarValueHandlerBase<T> <-
    _ArrayValueHandlerBase<T> <- _ValueHandler<T>

To simply:

_ValueHandlerBase <- _ValueHandler<T>

And the type-based special cases are handled locally within the methods of
_ValueHandler<T> via 'if constexpr()' rather than spread across the different
subclasses.

(Internal change: 2308134)