Breaking changes

• The blob class is now based on list_of(raw()) from the vctrs package (#11). This adds support for vec_cast() and vec_ptype2(). Some operations (such as subset assignment) are now stricter. The new_blob() constructor permits safe and fast construction of blob objects from a list, and validate_blob() checks an existing object for conformity with the rules.

• The new is_blob() deprecates the existing is.blob(). as.blob() is deprecated in favor of vec_cast() or the new as_blob() (which is just a thin wrapper around vec_cast()).

• Indexing a vector of blobs out of bounds now raises an error. Use NA as index to create a NULL blob.