Size constraint generation for native and VM

[ionathanch]

Nativeconv.native_conv_gen and Vconv.vm_conv_gen currently don't produce stage constraint sets from conversion like Reduction.gen_conv currently does. Will this be necessary? When are native and VM used? Does type checking etc. matter for them?

More notes:

• The main conversion function for Vconv is conv_whd. Definitions in Vmvalues will probably need to be changed, but changing this also means changing some C code, so maybe this is better left untouched.
• The main conversion function for Nativeconv is conv_val. Similarly, definitions in Nativelambda will probably need to be changed. I'm not sure if this is safe to touch.