The changes are mostly mechanical

- replace use of the blobcode vmod with vmod blob and adjust to new
  function signatures
- use struct vrt_blob for blobs and adjust to new struct members
- use separate pointers for generating output (blobs are const)
- use optional arguments for functions (minimal adjustments of
  existing code)
- replace WS_Reserve() with WS_ReserveAll()
- wipe needs the TRUST_ME() macro to un-const blobs

unresolved issues:

- padding errors lead to "vmod blob error: cannot encode, out of space",
  the respective tests have been moved to
  tests.disabled/padding_error.vtc

Update the docs to describe what affects how much secure memory may
be needed, and also to describe the limitations imposed by
PTHREAD_KEYS_MAX.

The previous implementation was not thread safe.
Now use pthread keys to create a handle for each object in each
thread, and then retrieve them in the same thread.

Had been checking an uninitialized length field if it is an exact
multiple of the block length.

- Fix an off-by-one error in ISO7816 unpadding.
- Unpadding errors are not assertion failures, handle the error.
- Check all the requirements of the padding scheme when unpadding.