Add a way to combine where with OR / AND. #158

[polo-asm]

Implementation for #158

Or / And condition can be written using 2 syntax :

User::where_col(|u| or( u.first_name.equal("John") , and( u.last_name.equal("Doe") , u.is_active.equal(true) ) ) )

User::where_col(|u| 
    u.first_name.equal("John")
    .or(  
        u.last_name.equal("Doe")
        .and(u.is_active.equal(true) ) 
    )
)

It's a proposal which can be enhanced. Maybe some documentation may be needed.

[polo-asm]

this using was not removed in my previous PR.

[polo-asm]

Needed to implement Clone & Copy in Schema structure

[polo-asm]

String prevent from implementing Copy. Because column and field are const it can be replaced by a static str

[polo-asm]

in closure |a| a.id.equal(0).or( a.name.equal("yyyy") ) the compiler must be able to copy a to use it twice. So I implemented Copy & Clone on the schema structure. I was also obliged to implement Copy on all query/clause structures.