Implement boxing/unboxing during VectorAPIExpansion under an option

- introduce new category of temps: vec_len_boxed_unknown
- build temp and whole webs as before
- wherever a temp was invalidated before assign vec_len_boxed_unknown instead
- store vectorization info for calls per node vs. per symbol reference for temps
- always vectorize nodes that are calls to vector API methods, even if input vectors
  need to be unboxed or result needs to be boxed
- since boxing/unboxing is not supported for all types, have an extra pass just before
  transformation and invalidate whole web if it contains unsupported boxing/unboxing
- transformation happens as before, except that we first check if children of non-vectorized
  node need to be boxed or children of vectorized node need to be unboxed, insert
  boxing/unboxing correspondingly
- this prototype is only enabled under -Xjit:enableVectorAPIBoxing option and the default
  behaviour shoulld be preserved