Move defaultProps resolution and type validation to the descriptor

This copies the propType and contextType validation to a wrapper around the
descriptor factory. By doing the validation early, we make it easier to track
down bugs. It also prepares for static type checking which should be done at the
usage site.

This validation is not yet active and is just logged using monitorCodeUse. This
will allow us to clean up callsites which would fail this new type of
validation.

I chose to copy the validation instead of abstracting it out to a common abstraction. This is just an
intermediate step to avoid spamming consoles. The original validation in the instance will be deleted as soon as we can turn on the warnings at the callsite. Copy+Delete makes this a more a much cleaner diff review/history.

Additionally, getDefaultProps are moved to become a static function which is
only executed once. It should be moved to statics but we don't have a
convenient way to merge mixins in statics right now. Deferring to ES6 classes.

This is still a breaking change since you can return an object or array from
getDefaultProps, which later gets mutated and now the shared instance is
mutated. Mutating an object that is passed into you from props is highly
discouraged and likely to lead to subtle bugs anyway. So I'm not too worried.

The defaultProps are now resolved in the descriptor factory. This will enable
a perf optimizations where we don't create an unnecessary object allocation
when you use default props. It also means that ReactChildren.map has access to
resolved properties which gives them consistent behavior whether or not the
default prop is specified.

This is a breaking change since it can affect how mapping over children and
transferPropsTo works together with defaultProps.