Abort decoding on a leading zero in a tag.

Thou shalt ISO/IEC 8825-1:2008 8.1.2.4.2 c):

    "bits 7 to 1 of the first subsequent octet shall not all be zero."
Merge pull request #3 from ALPHA-60/error-on-tag-leading-zero

Abort decoding on a leading zero in a tag.
re-refix #2, #3: overflows are hard
dox
small cleanup