Moved make_solver parameters into single params struct

This is an API breaking change!

This unifies treatment of amg and solver params in amgcl::make_solver,
amgcl::runtime::make_solver, and amgcl::mpi::subdmain_deflation classes.

Now, whenever a class contains both an amg hierarchy and an iterative
solver, the amg parameters may be accessed through the class
params::amg, and the solver parameters -- through params::solver.