Implement Proto transform for result type deduction

[ddemidov]

This will allow to achieve several things:

• [x] Automatic deduction of temporary terminal type.
• [x] Provide symmetric assign_multiexpression<OP,RHS,LHS> function similar to assign_expression<OP,LHS,RHS>.
• [x] Get rid of non-owning multivectors and replace those with vex::tuple (result of vex::tie). This in turn will allow to create writable multiexpressions (e.g. tied vector slices).
• [x] Solve issue #43, where either "abs" or "fabs" should be selected based on vex::abs argument type.

[ddemidov]

Done. I suspect there will be problems with OpenCL vector types (e.g. cl_double2), as std::common_type<T1,T2> (used in return type deduction) probably does not support those. It should be possible to provide a specification of std::common_type, but there could be other problems.

Closing this for now.