Safe curve element creation

[aleasims]

Methods __zkllvm_curve_*::from_coordinates are unsafe now.

Because not every pair of base coordinates belong to curve, we need to check this before creation. But how we can do it? And how llvm.assigner.curve.init intrinsic will react on invalid coordinates?

Also we propose to use TryFrom<(__zkllvm_field_X_base, __zkllvm_field_X_base)> interface for curve element creation.

[aleasims]

Waiting for instrinsic like llvm.assigner.curve.is_well_formed come to the LLVM and then implement safe curve initialization.