Mcsat api var order

[ahmed-irfan]

This PR :

• adds a method to set a variable ordering for making decisions in the MCSat search.
• moves the variable ordering from the mcsat options to the solver context.
• adds a api test case

[coveralls]

coverage: 65.14% (-0.004%) from 65.144%
when pulling 0dc69eb on mcsat-api-var-order
into 587bc83 on master.

[disteph]

"...make MCSAT decide the variables..."
Can we specify in the doc whether the ordering is forced during the initial search phase only, or if it is persistent across backtracks (i.e. variable activities will not affect the order)?

[ahmed-irfan]

good point. I will add that info