Update to support newest `3c` and Checked C `clang`

[mwhicks1]

Running this tutorial when using clang and 3c built from commit correctcomputation/checkedc-clang@2a0df40 yields some differences:

1. Instead of running into trouble with bignum_to_string in tests like golden.c, first, as it is presented in the README, the problem with bignum_from_string's use of &str[i] in bn.c and bn.h arises first. You can fix it just like the README says, though the original inferred type is not quite what is shown. When you do the fix, then the problems with bignum_to_string show up, and you fix those as prescribed.

2. This updated Checked C clang does not require printf, snprintf, etc. to be put in an _Unchecked block. As such, there are fewer unchecked blocks needed than are prescribed. Basically, only sscanf and the user-added bignum_from_string_using_strlen need to be in an unchecked block.