Issue #18: Test integer conversion in headers

https://github.com/alobbs/libhpack/issues/18

According to the draft there are several concerns regarding the VLQ
integer representation as explained in section **5. Security
Considerations**.

These 3 new tests verify that those concerns are addressed.

Integer concerns:
- size of integers in value: function indexed_big_value
- size of integers in encoded length: function indexed_many_zeroes

They also test that a very large, and incorrect, length value will not
break the library (function: literal_w_index_false_len).