## [REF] web,*: Centralize shallowEqual

Before this commit, 2 implementations of a "shallowEqual" function
coexisted, one in 'web/core/utils/objects.js' and another in
'web/core/utils/arrays.js'.

This commit only keeps the one in objects.js and makes the other simply
import and export it as well (easier to maintain while also easier to
find for those who want to work with arrays), while also taking an
optional comparison function argument in case the comparison needs to be
more specific (e.g. compare DateTime objects).
 
## [REF] web: Make datetime hook independant from props

This commit makes the datetime hook independant from the props update
mechanism it previously relied on (onWillStart & onWillUpdateProps) to
update its internal value.

This was an incoming issue as the future relational model will rely on
fine-grained reactivity to update the fields rather than updating the
model and re-rendering all child components (effectively calling
'onWillUpdateProps').

Now, the hook relies on an internal state that tracks whether the props
given by its caller changed from one render to another, which is check
at render time (= 'onWillRender').

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