Add basic fuzzing harness

[robmcl4]

Adds a basic fuzz harness based on the afl fuzzer.

This fuzzes: font load, font rasterization at fixed scale (60.0)

Fuzzed inputs include: character being rasterized, font bytes

[attackgoat]

This looks excellent, but the provided instructions fail for me with:

[-] Oops, can't find 'llvm-config'. Install clang or set $LLVM_CONFIG or $PATH beforehand.

@robmcl4 maybe a section on prerequisites? For me, on Ubuntu 20.10: apt install llvm. After that I got a c build error that appears to be drift from the 4/2020 change here.