Add three way lower_bound

[François Dumont]

Hi

Following the message to propose an alternative lower_bound and the 
reply to use three way comparison I try to implement this.

Before going further I wonder if this is something possible ?

The purpose of the:

if constexpr (three_way_comparable<>)

is to make sure that we use it only if there is a proper <=> operator 
defined. Afai understood what is in <compare> we can have the 
__synth3way for any type as long as < exist. But I think that if <=> is 
implemented in terms of < then it might be too expensive, the actual 
lower_bound might already be implemented this way.

My main concerns is of course Standard conformity, could it be ok ?

François

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