Botan's fuzzers now have builtin maximums where necessary, instead of relying on
max len option. So the code to generate the .options files goes. Suggested by
@kcc in #150
Use corpus for the common formats, seems to help with initial coverage at least.
Disables Botan's pool allocator at build time, since it hides things from ASan.
The fuzzer driver already tries to disable it at runtime in LLVMFuzzerInitialize,
but the Clusterfuzz coverage report indicates that this init function is not
ever called, and the pool allocator ends up being used.