Use the test-many-sizes framework for string writes as
well (previously, it was only used for booleans and integers). This
way, more edge cases are tested with less test code.
This commit removes buffer overwrite checks. Instead of these checks,
run the test suite under a memory sanitizer (which we do in our CI).
Omit negative integers and MPIs that would result in values that look
like negative INTEGERs, since the library doesn't respect the
specifications there, but fixing it has a serious risk of breaking
interoperability when ASN.1 is used in X.509 and other
cryptography-related applications.
